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COLOUR    VISION 


?vv<x\versity  of  /» 
THE    TYNDALL    LECTURES 


COLOUR 


BfiING 


DELIVERED    IN    1894 

AT 


THE  ROYAL  INSTITUTION 


CAPT.  W.  DE  W.^BNEY,  C.B.,  D.C.L.,  F.R.S. 


LATE   ROYAL   ENGINEERS 


WITH  COLOURED  PLATE  AND  NUMEROUS  DIAGRAMS 


NEW  YORK 

wiivJtJAM  'WppD^:  ;g:ompany 


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^9r 


CONTENTS. 


PAGE 

Pkeface .       vii 

CHAPTER  I. 
The  Eye 1 

CHAPTER  II. 
Simple  Colours  and  their  Mixture 15 

CHAPTER  III. 
Three  Colour  Sensations  Possible 32 


I 


CHAPTER   IV. 
he  Young  and  Bering  Theories  of  Colour  Vision        ...       41 


CHAPTER  V. 
General  Aspect  of  Colour  Blindness 58 

CHAPTER  VI. 

Colour  Blindness  exhibited  by  Colour  Discs  and  exhibited  by 

Luminosity  Curves  of  the  Spectrum 74 

CHAPTER  VII. 

Luminosity  of  Colours  to  Different  Parts  of  the  Retina      .      .       88 


VI  Contents. 

CHAPTER  yill. 

Luminosity  of  a  Feeble  Spectrum  and  the  Limit  of  the  Per- 
ception of  Colour 98 

CHAPTER   IX. 
The  Extinction  of  Light  from  the  Spectrum 108 

CHAPTER  X. 
The  Extinction  of  the  Perception  of  Light  by  the  Colour  Blind     122 

CHAPTER  XL 
Tobacco  Blindness 137 

^.           CHAPTER  XII. 
Examples  of  Colour  Blindness  due  to  Disease 148 

CHAPTER  XIII. 

The  Holmgren  Test  for  Colour  Blindness 167 

CHAPTER  XIY. 

The  Spectrum  Test  for  Colour  Blindness 180 

CHAPTER  XV. 
The  Young  and  Hering  Theories  of  Colour  Vision  Compared  ,     187 

Appendix 201 

Index .     229 


PREFACE, 


'  I  'HE  writer  had  for  some  years  past,  in  conjunc- 
tion with  General  Festing,  and  recently  as 
Secretary  and  Member  of  the  Colour  Vision  Com- 
mittee of  the  Royal  Society,  carried  out  a  series 
of  investigations  on  colour  vision,  and  selected  that 
subject  when  he  was  invited,  in  1894,  to  deliver  the 
Tyndall  Lectures  at   the  Royal  Institution. 

The  brief  time  allotted  for  these  lectures — an  hour 

on    three    successive    Saturday    afternoons — restricted 

the   discussion  of  some   aspects   of   the  question,  and 
confined  its  treatment   in   the  main  to  those  features 

Kost  readily  explicable  by  the  physicist,  and  to 
inging  into  notice  the  latest  results  which  had  been 
>tained    from    physical    experiments.      How  J  far    the 

364 


viii  Preface. 

writer  has  succeeded  in  the  task  which  he  then   out- 
lined it  is  for  the  reader  to  determine. 

There  was  no  intention  in  the  first  instance  to 
publish  these  lectures.  After  their  delivery,  many 
persons  expressed  a  desire  that  the  information  they 
contained  should  be  rendered  accessible  to  such  as 
were  interested  in  the  theory  of  colour  vision,  and 
in  deference  to  that  desire  the  lecture-notes  have 
been  re-cast  in  book  forrn.  For  the  reader's  con- 
venience the  matter  is  now  divided  into  chapters 
instead  of  into  lectures,  and  a  few  additions  have 
been  made  in  the  text  to  explain  some  of  the  ex- 
perimental work  to  those  who  have  not  facilities  for 
its  repetition. 

The  writer  has  to  acknowledge  several  debts  of 
gratitude.  First,  to  Mr.  E.  Nettleship,  for  his  kindness 
in  looking  over  the  proofs,  and  making  valuable  sug- 
gestions whilst  the  work  was  passing  through  the 
press ;  and  also,  as  will  be  seen  throughout  its  pages, 
for  many  of  the  interesting  cases  of  defective  colour 
perception  which  have  been    examined   by  the   some- 


Preface.  ix 

what  novel  methods  described.  Next,  the  writer's 
oratitude  is  due  to  Professor  M.  Foster  for  the  per- 
mission he  has  given  to  use  his  admirable  description 
«)f  the  Hering  theory ;  and,  lastly,  to  the  Koyal  Society 
for  the  permission  it  accorded  to  use  various  diagrams 
which  have  served  as  illustrations  to  papers  which 
have  appeared  in  its  "  Philosophical  Transactions " 
and  "  Proceedings." 


^    /  "^"1     LI  BR  A  '- 


COLOUR   VISION. 


CHAPTER     I. 

T  MUST  commence  this  course  by  saying  that  I  feel 
the  honour  that  has  been  done  me  in  asking  me 
to  undertake  it,  connected  as  it  is  with  the  name  of 
Tyndall,  whose  recent  removal  from  our  midst  has 
been  deplored  by  all  lovers  of  science,  and  by  none 
more  than  by  those  who  have  had  the  privilege  of 
listening  to  him  at  this  Institution.  It  is  my  duty 
to  speak  on  some  subject  of  physics,  and  the  subject 
I  have  chosen  is  Colour  Vision.  I  hope  it  will  not 
be  considered  inappropriate,  since  it  was  Thomas 
Young,  the  physicist,  whose  connection  with  this 
Institution  is  well  known,  who  first  propounded  a 
really  philosophical  theory  of  the  subject.  Interesting 
as  it  may  be  to  trace  how  old  theories  have  failed  and 

B 


2  Colour   Vision, 

new  ones  have  started,  I  feel  that  for  those  who,  like 
myself,  have  but  little  time  at  command  in  which  to 
address  you,  the  historical  side  of  this  question  must 
of  necessity  be  treated  incompletely. 

Colour  vision  is  a  subject  which  enters  into  the 
domains  both  of  physics  and  physiology,  and  it  is 
thus  difficult  for  any  one  individual  to  treat  of  it 
exhaustively  unless  he  be  a  Helmholtz,  who  was  as 
distinguished  in  the  one  branch  of  science  as  he  was 
in  the  other.  I  am  not  a  physiologist,  and  at  the 
most  can  only  pretend  to  an  elementary  knowledge 
of  the  physiology  of  the  eye,  but  I  trust  it  is 
sufficient  to  prevent  myself  from  falling  into  any 
grievous  error.  I  shall  try  and  show  you,  however, 
that  the  subject  is  capable  of  being  made  subordinate 
to  physical  methods  of  examination.  I  must  neces- 
sarily commence  by  a  very  brief  description  of  those 
parts  of  the  eye  in  which  it  is  supposed  the  seat  of 
vision  lies,  but  in  terms  wdiich  are  not  too  technical. 
As  to  the  mere  optical  properties  of  the  eye  I  shall  say 
but  little,  for  they  are  not  necessary  for  my  purpose,  j 
although  more  particularly  adapted  to  mathematical 
treatment  than  the  other  properties  1  have  to  describe.    ! 

The    eye   may   be    diagrammatically  represented  as 
in  the  figure  which   is  supposed   to    be   a    horizontal 


The  Eye. 


Fig.  1. 


section  of  it,  the  figure  being  reproduced  from  Professor 
Michael  Foster's  Physiology. 

As  far  as  the  perception  of  colour  is  concerned,  the 
principal  part  of  the  eye  which  is  not  distinctly  optical 
— i.e.  for  the  produc- 
tion of  images  —  is 
the  retina,  and  this 
it  will  be  seen  is  in 
reality  an  outcrop  of 
the  brain,  the  connec- 
tion between  the  two 
being  the  optic  nerve. 
Owing  to  this  connec- 
tion, it  is  not  easy  to 
determine  where  the 
seat  of  colour  percep- 
tion is  located  ;  but 
for  the  purpose  of 
physical  investigation 
this    is   not  of  first- 


8cl  is  the  sclerotic  coat.  Ch  the  choroid  coat, 
with  CP  the  ciliary  process.  I  is  the  body 
of  the  Iris.  H  is  the  retina  or  inner  wall. 
TE  the  pigment  epithelium  or  outer  wall. 
Ij  the  lens  held  by  th«  suspensory  ligament 
«2>.Z.  YH.  is  the  vitreous  humour.  0^  the 
optic  nerve,  ox  is  the  optic  axis,  in  this 
case  made  to  pass  through  the  fovea  cen- 
tralis,/.c. 


rate  importance,  nor 
does  it  afi'ect  the  discussion  of  rival  theories  except 
in  a  minor  degree.  There  are  other  subsidiary  adjuncts 
in  the  eye  to  which,  however,  I  must  call  attention,  as 
they  have  a  distinct  bearing  on  the  apparent  intensity 


B    2 


4  Colour   Vision. 

of  some  colours  and  of  the  hue  that  mixtures  of 
others  are  perceived.  The  first  is  what  is  called  the 
'*  macula  lutea,"  or  yellow  spot,  a  spot  which  it  may 
be  assumed  exists  in  every  eye.  It  is  horizontally 
oval  in  form,  and  is  situated  in  the  very  centre  of 
the  retina,  embracing  some  6°  to  8°  in  angular  mea- 
sure. It  has  a  brownish  or  yellowish  tint,  and  the 
retina  at  this  part  is  slightly  depressed,  being  bounded 
by  a  slightly  raised  rim.  In  the  centre  of  this  area 
the  retina  becomes  very  thin,  having  a  depression 
about  y^o"  of  an  inch  or  '3  millimetres  in  diameter, 
which  is  named  the  "  fovea  centralis,"  where  it  is 
said  that  vision  is  the  most  acute.  This  statement 
can  be  well  credited  when  we  come  to  consider  where 
the  seat  of  the  stimulation  of  sensation  lies.  The 
colour  which  tints  the  yellow  spot  is  strongest  at  the 
crater-like  rim,  and  fades  away  centrally  and  periphe- 
rally, and  is  said  to  be  wholly  absent  in  the  fovea 
centralis. 

As  the  colour  of  this  spot  is  yellow  or  brown  in 
the  living  eye  (and  that  it  is  probably  brown  the 
absorption  indicates),  it  follows  that  white  light  passing 
through  it  must  be  deprived  of  some  of  its  compo- 
nents, though  in  differing  degrees.  If  the  seat  of 
sensation  is  at  the  outer  layer  of  the  retina,  as   we 


The  Yellow  Spot.  5 

shall  shortly  see  must  be  the  case,  it  will  further  be 
seen  that  when  light  of  any  colour  which  the  brown 
pigment  will  absorb  more  or  less  completely  falls  on 
different  parts  of  the  oval  area,  the  absorption  must 
vary  at  each  part,  and  the  intensity  of  the  perceived 
ligiit  will  be  least  at  the  rim  and  increase  centrally  and 
peripherally.  As  the  centre  of  the  yellow  spot  or  fovea 
is  coincident  approximately  with  the  point  where  the 
axis  of  the  eye  cuts  the  retina,  the  image  of  an  evenly 
illuminated  object,  when  looked  at  directly,  must  fall 
on  the  yellow  spot.  If,  therefore,  a  patch  of  such 
light,  the  image  of  which  more  than  covers  the  spot, 
be  observed,  it  ought  to  exhibit  a  varying  brightness  of 
colour  corresponding  to  the  strength  of  the  colouring 
matter  which  exists  at  the  different  parts.  This  it 
but  rarely  does,  for  habit  and  constant  interpretation 
of  what  should  be  seen  prevents  the  mind  from 
distinguishing  these  variations ;  but  if  the  colour 
brightness,  as  perceived  by  the  different  parts,  be 
submitted  to  measurement  by  proper  means,  the  varia- 
tions in  brightness  of  the  image  can  be  readily 
recognised.  A  very  common  method  of  exhibiting 
the  presence  of  the  pigment  is  to  look  at  a  bright 
white  cloud  through  a  layer  of  chrome  alum.  Chrome 
alum   transmits   red    and  blue  -  green    rays.      Now   as 


6  Colour  Vision. 

the  spectrum-blue  rays  are  those  which  the  pigment 
will  absorb,  it  follows  that  the  colour  of  the  solution 
should  appear  ruddy  to  the  central  part  of  the  eye, 
but  on  the  rest  of  the  retina  it  should  appear  of  its 
ordinary  purplish  colour.  At  a  first  glance,  and  before 
the  eye  has  become  fatigued,  this  is  the  case,  but  the 
phenomenon  soon  disappears.  Another  way  of  forming 
an  idea  as  to  what  the  yellow  spot  absorbs  is  to  throw 
a  feeble  spectrum  on  a  white  surface  and  cause  the 
eye  to  travel  along  it.  If  the  spectrum  be  viewed  so 
that  it  does  not  occupy  more  than  about  40°  of  the 
retina,  the  movement  of  the  eye  will  show  a  dark 
band  travelling  along  the  green,  blue,  and  violet  regions 
as  the  image  of  these  parts  of  the  spectrum  fall  on  the 
yellow  spot,  and  their  apparent  brightness  will  increase 
as  they  fall  outside  the  absorbing  area.  This  proves 
that  an  absorption  takes  place  in  this  area. 

The  retina  consists  essentially  of  an  inner  and  outer 
wall,  enclosing  matter  w^hich  is  similar  to  the  grey 
matter  of  the  brain.  On  the  inner  wall  are  the 
vessels  which  are  connected  with  the  optic  nerve. 
The  outer  wall  is  epithelium  coloured  with  a  pig- 
ment, and  it  is  here  that  the  visual  impulses  begin, 
although  the  rays  of  light  giving  rise  to  them 
have    tO;  pass    through    the    thickness    of    the    retina 


Purkinjes  Figures.  7 

before  so  doing.  It  has  already  been  stated  that 
the  light  has  to  pass  through  the  thickness  of  the 
yellow  spot  before  a  visual  sensation  is  felt  in  the 
centre  of  the  field,  and  the  experiments  just  given 
offer  a  fair  proof  of  the  truth  of  the  assertion,  but 
there  is  still  another  which  is  perhaps  more  conclusive. 
Suppose  we  have  a  hollow  reflecting  ball,  as  shown 
in  Fig.  2,  and  through  an  orifice  A  we  project  a 
beam  of  light  to  B,  which  meets 
an  obstruction,  S,  in  its  path,  then 
A  B  would  be  reflected  from  B  to 
C  on  a  screen  C  F,  and  the  ob- 
struction S  would  be  marked  at  C. 
If  another  beam  from  D  was  di- 
rected so  as  to  meet  the  same 
obstruction,  its  presence  would  be 
marked  at  F.  Knowing  the  distance  of  the  centre 
0  of  the  hollow  sphere  from  F  C  and  its  diameter, 
and  measuring  the  distance  between  F  and  C  and 
their  respective  distances  from  the  axis  of  the  sphere, 
the  distances  S  B  and  S  E  can  be  calculated.  This 
method  is  applied  in  the  formation  of  what  are  known 
as  Purkinje's  figures.  The  simplest  case  is  where  a 
beam  of  light  is  directed  through  the  sclerotic  and 
transmitted  through  the  lens.      Images  of  the   retinal 


8  Colour   Vision. 

vessels  are  distinguished  as  at  S,  and  it  is  found 
that  they  cast  shadows,  which  are  seen  as  dark  lines 
in  the  glare  of  the  field  of  vision.  The  sensation 
of  light  must  therefore  come  from  behind  these 
vessels,  and  calculation  shows  that  the  seat  of  the 
sensation  is  close  to  the  pigmented  inner  wall  of  the 
retina. 

Lying  here  is  a  layer  of  what  are  known  as  rods 
and  cones,  which  have  a  connection,  either  actual  or 
functional,  with  the  optic  fibres  which  largely  compose 
the  inner  wall  of  the  retina,  and  are  connected  with 
the  optic  nerve.  In  the  yellow  spot  the  cones  are 
much  more  numerous  than  the  rods,  but  in  the  peri- 
pheral part  the  reverse  is  the  case.  In  the  fovea  the 
rods  appear  to  be  altogether  absent.  The  total  number 
of  cones  in  the  eye  has  been  calculated  to  be  about 
3,000,000,  of  which  about  7,000  are  in  the  small  fovea. 
The  number  of  cones  will  give  an  idea  of  their  dimen- 
sions. This  detail  has  been  entered  into  as  it  has  been 
supposed  that  these  rods  and  cones  are  all-important 
in  translating  light-waves  into  visual  impulses.  The 
inner  wall  of  the  retina  of  most  human  eyes,  as  has 
been  mentioned,  is  stained  with  a  black  pigment, 
fuscin,  though  in  albinos  it  is  absent.  What  its  par- 
ticular use  may  be  is  still  unknown,  for  its  change  by 


Visual  Purple.  9 

light  is  so  slow  that  it  can  scarcely  be  the  cause  of 
vision.  In  the  outer  parts  of  the  rods  is,  however, 
diffused  a  substance  highly  sensitive  to  light,  called 
the  "  visual  purple,"  from  its  colour,  and  a  theory 
founded  on  chemical  action,  produced  by  a  change 
in  this  substance,  has  been  promulgated.  Fascinating, 
however,  as  such  a  theory  must  be,  it  lacks  confirma- 
tion. The  fact  that  the  cones  do  not  contain  it, 
and  that  in  the  fovea  are  cones  alone,  renders  it 
difficult  to  reconcile  the  theory  with  the  fact  that 
this  part  of  the  retina  possesses,  we  are  told,  the 
greatest  acuteness  of  sensation  as  res^ards  lio^ht  and 
colour.  - 

The  eyes  of  most  vertebrate  animals,  it  may  be 
remarked,  have  this  visual  purple,  but  in  those  of 
the  bat,  owl,  hen,  and  some  others  the  colouring 
matter  seems  to  be  absent.  Visual  purple  is  an  in- 
teresting substance,  however,  and  as  it  is  found  in 
the  eye  it  probably  exercises  some  useful  function, 
though  what  that  function  may  be  is  at  present 
unknown.  That  images  of  objects  can  be  formed  on 
the  retina,  owing  to  the  bleaching  of  this  substance, 
has  been  proved  by  experiment.  The  purple  is  first 
changed  to  a  yellow  colour,  and  then  passes  into 
white.     These    "  optograms,"   as   they   are   called,   can 


I 


lO  Colour  Vision, 

be  fixed  in  an  excised  eye  if  the  retina  be  detached, 

and  then  be  treated 
with  a  weak  solution 
of  alum. 

Many  persons  are 
not  aware  of  the 
extent  of  the  field 
of  view  which  the 
eye  embraces.  Ver- 
tically it  takes  in 
about  100°,  whilst 
horizontally  it  will 
take  in  some  145°, 
more  or  less.  The 
field  is  smaller  on 
the  nasal  than  on 
the  temporal  side. 
When  both  eyes  are 
used,  the  combined 
field  of  view  is 
larger  horizontally, 
being  about  180°. 
The  field  of  view 
which    is    common 

to  both  eyes  is  roughly  a  circle  of  about  90°.     There 


Distinctness  of  Vision.  1 1 

is,  however,  a  marked  difference  in  the  distinctness 
with  which  objects  are  perceived  in  the  different  parts 
of  field  of  view.  On  the  fovea  centralis  two  dots  placed 
so  as  to  subtend  an  angle  of  60"  will  be  perceived  as 
double.  That  is  to  say,  if  a  piece  of  paper,  on  which 
are  two  dots  -3^^  of  an  inch  apart,  be  placed  10  feet 
away  from  the  observer,  these  dots  will  be  seen  as 
separated,  whilst  dots  (in  this  case  they  should  be 
black  and  of  good  dimensions)  placed  half  -  an  -  inch 
apart  would  still  appear  as  one  if  viewed  at  the  same 
distance  near  the  periphery  of  the  retina.  In  the 
yellow  spot  the  distance  apart  of  the  cones  is  such 
that  they  subtend  about  the  same  angle  as  the  dots 
when  they  are  seen  separate,  viz.,  about  60";  that  is, 
they  are  about  ^q^qqq  of  an  inch  apart,  and  hence 
may  have  something  to  say  to  the  limit  of  separation. 
The  field  for  the  perception  of  colour  is  different  to 
that  for  light. 

The  diagrams  (Fig.  3)  will  show  the  fields  in  a 
satisfactory  manner.  The  concentric  circles  are  sup- 
posed to  be  circles  lying  on  the  retina  corresponding 
to  parallels  of  latitude  on  a  globe,  and  are  not,  there- 
fore, equi-distant  when  seen  in  projection.  To  make 
these  circles  it  must  be  imagined  that  we  have  a  bowl, 
in  the  middle  of  which  is  a  thin  rod  standing  upright 


1 2  Colour   Vision, 

and  passing  through  the  centre,  and  another  rod 
attached  to  it  at  the  centre  of  the  sphere  of  exactly 
the  length  of  the  radius.  If  this  last  arm  be  opened 
to  make  an  angle  of  5°  with  the  fixed  rod,  and  Ije 
twisted  round  like  the  leg  of  a  compass  against  the 
bowl,  it  will  make  a  circle,  the  projection  of  which  will 
give  the  innermost  circle  of  the  diagram  ;  if  opened 
to  10°  it  will  give  the  next  circle,  and  so  on  for  every 
subsequent  10°.  The  lines  passing  through  the  centre 
are  30°  from  one  another,  the  line  stretching  from 
360°  to  180°  being  a  line  supposed  to  be  vertical. 
By  means  of  an  instrument  called  the  perimeter,  the 
field  of  vision  for  each  eye  can  be  measured.  With 
its  aid  any  small  object  can  be  made  to  fall  on  any 
part  of  the  retina  by  directing  the  axis  of  the  eye 
to  a  fixed  point  and  moving  the  object  along  one 
of  the  diameters.  Suppose  we  wish  to  ascertain  the 
field  for  a  white  object,  a  small  white  disc  is  moved, 
say,  along  the  horizontal  line,  and  the  angles  at  which 
the  retina  just  no  longer  sees  it  are  noted.  This  gives 
two  points  in  the  field,  and  they  are  plotted  on  the 
chart — in  Fig.  3  one  touches  the  outside  circle,  and  the 
other  is  at  an  angle  of  about  65°.  The  field  of  vision  is 
next  tested  along  another  line,  say  300°  to  120°,  and 
other  points  noted  and  marked  on  the  chart.      When 


Colour  Fields,  13 

the  whole  circle  has  been  examined,  the  various  points 
are  joined  together,  and  we  have  the  boundary  of 
vision  for  a  white  object.  The  boundaries  of  the  colour 
perception  for  (say)  small  red  and  green  discs  are  found 
in  the  same  way.  The  former  is  depicted  in  the  left- 
hand  chart  and  gives  the  field  for  the  right  eye,  and 
the  latter  with  that  for  white  in  the  right-hand  chart 
for  the  same  eye.  It  will  be  noticed  that  two  boun- 
daries are  given,  one  taken  at  mid-day  and  the  other 
at  6  p.m.  The  brighter  the  colour,  the  larger  is  the 
boundary  in  both  cases,  showing  that  the  field  of  colour 
vision  varies  according  to  the  illumination.  Now  it  is 
difficult  from  this  method  of  experimenting  to  determine 
whether  the  fields  for  different  colours  are  the  same 
or  differ  in  extent,  as  we  have  no  information  as  to 
whether  the  colours  themselves  which  were  used  were 
physiologically  equal.  The  only  way  by  which  this  can 
be  satisfactorily  determined  is  by  using  spectrum  colours 
each  of  known  brightness  and  area.  (Some  preliminary 
experiments  made  by  myself  regarding  the  colour  fields 
w^ill  be  found  in  the  appendix,  and  will  be  referred  to 
later.)  It  must  not  be  thought  that  the  various  colour 
boundaries  mark  the  limit  at  which  light  is  perceived, 
but  only  the  limit  at  which  colour  is  seen  ;  outside  the 
boundaries  the  objects  appear  of  a  nondescript  colour, 


14  Colour  Vision. 

to  which  we  shall  by-and-by  call  attention.  The  yellow 
spot  lies  within  the  circle  of  5°,  and  the  blind  spot  on 
which  no  sensation  of  light  is  stimulated  is  shown  by 
the  black  dot  about  15°  away  from  the  centre. 

I  have  only  attempted  to  sketch,  in  unphysiological 
language,  the  primary  apparatus  with  which  our 
experiments  in  colour  have  perforce  to  be  made. 


(   15  ) 


CHAPTER     11. 

It  will  be  seen,  then,  that  in  measuring  colour  or 
light  several  circumstances  have  to  be  taken  into 
account.  These  are  not  simple,  and  require  differen- 
tiating one  from  another  before  the  results  of  colour 
measures  can  be  finally  laid  down  as  correct,  or  as 
being  held  to  be  applicable  to  all  cases. 

We  must  naturally  ask,  w^hat  is  colour  ?  The  answer 
I  should  like  to  pass  over  entirely.  It  can  only  be 
described  as  a  sensation,  just  as  we  should  describe 
touch  as  a  sensation.  It  has,  however,  one  advantage 
over  most  sensations,  in  that  it  is  a  sensation  which 
can  be  submitted  to  empyric  measurement.  The 
question  whether  certain  phenomena,  such  as  the 
colours  produced  by  simultaneous  contrast,  are  sub- 
jective or  real,  does  not  require  answering  for  the 
purpose  that  we  have  in  view,  but  the  results  recorded 
may  probably  help  to  throw  light  on  it.  Colour  is 
an  impression   caused   by   the  stimulation   in  the  eye 


1 6  Colour   Vision, 

of  some  apparatus,  that  lies  near  the  outer  wall  of 
the  retina,  the  effect  of  the  stimulation  being  con- 
veyed by  the  optic  nerve  to  the  brain.  If  this 
apparatus  be  complicated  by  being  made  up  of 
distinct  parts,  each  of  which  transmits  its  own  kind 
of  impression  to  the  brain,  it  is  not  only  quite 
possible,  but  more  than  probable,  that  when  one  part 
is  absent  or  injured  the  particular  impression  for  which 
it  is  responsible  will  be  lacking,  and  that  the  sum  of 
the  impressions  due  to  the  remainder  will  be  unlike 
that  perceived  when  they  are  all  working  together. 

In  every  investigation,  whether  it  be  in  physical  or 
In  any  other  branch  of  science,  it  is  better  to  work 
up  from  the  simple  to  the  more  complicated ;  and 
acting  on  this  plan,  it  is  better  to  commence  experi- 
menting wdth  simple  rather  than  with  complex  colours, 
though  they  may  apparently  produce  precisely  the 
same  sensations.  I  shall,  with  this  in  view,  devote 
most  of  the  remaining  part  of  this  chapter  to  some 
necessary  experiments  wdth  simple  colours.  The  simple 
colours  are  those  of  the  spectrum,  and  are  the  result 
of  motion  in  the  ether,  which  pervades  all  space.  The 
motion  is  in  the  form  of  undulations  or  waves,  and 
each  colour  is  due  to  a  series  of  these  waves,  which 
have  a  definite  length.     Thus,  6562  ten-million ths  of 


Wave- Lengths  of  Light.  1.7 

a  millimetre  produces  to  most  of  us  a  red  colour  in 
the  spectrum  (see  Plate  L),  occupying  the  position 
indicated  by  a  black  line  known  as  the  C  line  in  the 
solar  spectrum. 

A  table  of  wave-lengths  of  certain  lines  in  the  solar 
spectrum  is  given  below  : — 

TABLE  OF   WAVE-LENGTHS   IN   TEN-MI LLIONTHS 
OF   A   MILLIMETKE. 


B,  deep  red 

6866 

b,  green  . 

5183 

Lithium,  cherry  red 

6705 

F,  bluish  greeii 

4861 

C,  red      . 

6562 

Lithium,  blue  . 

4603 

D,  orange 

5892 

G,  violet. 

4307 

E,  green  . 

5269 

H,  extreme  violet 

3968 

The  rays  in  the  different  parts  of  the  spectrum 
being  due  to  these  simple  vibratory  motions,  cannot 
be  decomposed  further.  And  it  makes  no  matter 
whether  we  see  them  as  different  colours  or  not,  they 
will  always  issue  at  the  same  angle  from  the  same 
prism  (if  the  prism  be  used  to  form  the  spectrum), 
when  it  is  turned  to  the  same  angle  to  the  incident 
light.  Milestones  are  useful  along  a  road  to  tell  us 
where  we  are  in  reference  to  some  central  place,  and 
these  black  lines  in  the  spectrum  serve  the  same  end. 
But  they  have  the  advantage  over  the  milestone,  for 
whilst  the  last  will  tell  us  how  far  we  are  from,  say, 
York  or  London,  the  former  tell  us  our  distance  from 

c 


1 8  Colour  Vision. 

a  zero  point.  We  thus  have  a  scale  of  light  of  different 
wave-lengths  laid  down  for  us,  which  we  can  apply 
to  the  study  of  the  sensations  stimulated  in  the  eye, 
and  so  have  the  means  of  instituting  a  comparison 
between  the  colour  vision  of  different  eyes.  A  mixed 
or  composite  colour  is  in  a  different  category,  however, 
to  the  simple  colour,  as  you  will  see  directly.  It  is 
one  which  may  be  formed  by  any  number  of  rays  of 
different  wave-lengths  falling  on  the  eye.  What  these 
rays  are  we  can  only  tell  by  analysing  the  light  and 
referring  them  to  the  spectrum. 

The  instrument  before  you  is  one  which  I  have 
used  before  in  this  theatre ;  but  as  the  major  part 
of  my  experiments  have  been  carried  out  with  it,  in 
case  those  who  are  present  may  not  be  acquainted 
with  it,  it  will  be  necessary  to  describe  it  very  briefly. 
The  general  arrangement  of  the  apparatus  is  given  in 
the  accompanying  diagram,  Fig.  4. 

R  R  are  rays  coming  from  the  source  of  light,  be  it 
sun  light  or  the  electric  light,  and  an  image  of  the  one 
or  the  other  is  formed  by  a  lens  L^  on  the  slit  Si  of 
the  collimator  C.  The  parallel  rays  produced  by  the 
lens  L2  are  partially  refracted  and  partially  reflected. 
The  former  pass  through  the  prisms  Pi,  P2,  and  are 
focussed  to  form  a  spectrum  at  D  by  a  lens  L3.     D  is  a 


Colour  Patch  Apparatus. 


19 


movable  screen  in  which  is  an  aperture  S._,,  the  width 
of  which  can  be  varied  as  desired.     The  rays  are  again 


Fig.  4. 


//   A. 


11 

I 


M 


1   JE 


collected  by  a  lens  L4,  and  form  a  white  image  of  the 
surface  of  the  last  prism  on  the  screen  E.  If  the  light 
passing  through   S2  is    alone  used,   the  image  at  E  is 

0   2 


^O  Colour    Vision, 

formed  of  practically  mono-chromatic  light.  Part  of 
the  rays  falling  on  Pi  are,  as  just  said,  reflected,  but) 
as  it  and  the  refracted  part  are  portions  of  the  light 
passing  through  the  slit  S^,  they  both  must  vary  pro- 
portionally. If  then  we  use  the  reflected  portion  as 
a  comparison  light  to  the  spectrum  colours,  the  rela- 
tive intensities  of  the  two,  though  they  may  vary 
intrinsically,  will  remain  the  same.  The  rays  reflected 
from  Pi  fall  on  Gr,  a  silver  or  glass  mirror,  and,  by 
means  of  another  lens  L^,  also  can  be  caused  to  form 
a  white  patch  on  the  screen  E,  alongside  the  patch 
of  colour.  At  M,  or  anywhere  in  the  path  of  the 
beams,  an  electro-motor  driving  a  sector  with  aper- 
tures which  can  be  opened  or  closed  whilst  rotating, 
is  placed,  and  the  illumination  of  either  beam  can  be 
altered  at  will.  To  obtain  a  large  spectrum  on  the 
screen  E,  all  that  is  necessary  is  to  interpose  a  lens 
of  fairly  short  focus  in  front  of  L4,  when  a  spectrum 
of  great  purity  and  brightness  can  be  formed. 

If  it  be  required  to  measure  the  width  of  the  slits  S2 
(which  we  shall  see  further  on  is  often  necessary),  a 
small  lens  of  short  focal  .length  placed  behind  L4  and 
near  the  slit  will  cast  a  magnified  image  on  E,  and  by) 
means  of  a  scale  placed  there,  the  widths  of  each  slit,: 
if  there   are   more   than   one,   can   be   read   ofl*  on  the 


p° 


scale  by  bringing  them 
successively  into  the 
same  colour. 

Originally  the  com- 
parison light  was  a 
candle,  and  it  answered 
its  purpose  fairly  well, 
and  for  obtaining  abso- 
lute measures  is  con- 
venient at  the  present 
time.  Fig.  5  will  show 
its  arrangement,  but  as 
both  the  candle  and  the 
electric  light  may  vary 
independently  of  each 
other,  it  will  be  seen 
that  for  merely  the 
comparison  of  the  dif- 
ferent spectrum  colours, 
the  previous  arrange - 
ent  is  the  better.  In 
both  cases  the  two  beams 
—  the  direct  and  the 
comparison  —  may  be 
made    to    cast    shadows 


Colour  Patch  Apparains. 

Fig.  5. 


21 


I 


.2  2  Colour   Vision. 

by  placing  a  rod  in  their  path,  the  shadow  cast 
by  one  light  is  then  illuminated  by  the  other  light. 
By  moving  the  rod  towards  or  from  the  screen  the 
shadows  can  be  brought  side  by  side. 

With  this  instrument  it  is  easy  to  demonstrate  that 
a  mixed  colour  may  be  mistaken  for  a  simple  colour 
of  the  spectrum.  In  a  glass  cell  with  parallel  sides 
is  a  solution  of  potassium  bichromate,  which,  to  myself 
and  probably  most  of  you,  has  a  beautiful  orange  colour. 
The  spectrum  of  white  light  is  now  on  the  screen,  and 
if  this  orange  liquid  is  placed  in  the  path  of  the  white 
light  before  it  reaches  the  prisms,  all  the  violet,  blue, 
and  most  of  the  green  is  cut  off,  leaving  some  green- 
yellow,  orange  and  red  only  on  the  screen.  That 
these  form  the  orange  colour  of  the  bichromate  is 
readily  shown  by  removing  the  auxiliary  lens.  The 
spectrum,  which  has  its  focus  at  D,  is  now  recombined 
into  a  patch  of  light,  which  is  at  once  seen  to  be  the 
colour  of  the  solution. 

The  colour  of  the  bichromate  is  therefore  a  complex 
or  mixed  colour  according  to  our  definition,  for  it  is 
made  up  of  a  large  number  of  simple  colours.  What  I 
desire  to  show,  however,  is  that  this  complex  colour  can 
be  mistaken  by  the  eye  for  a  simple  colour.  First,  let 
US  interpose  the  cell  with  the  bichromate  in  the  path 


Simple  and  Complex  Colo7trs.  23 

of  the  reflected  beam,  and  throw  the  patch  of  light 
formed  by  it  on  a  white  surface  A  (Fig.  6),  alongside 
the  patch  of  light  B  formed  by  the  spectrum.  Next 
let  us  pass  a  single  aperture  (Fig.  7),  which  can  be 
opened  and  closed  by  a  screw  arrangement,  through 
the  spectrum.  By  careful  movement  we  at  length 
come  to  an  orange  ray,  which  is  spread  out  by  the 
apparatus  to  form  a  patch  on  B,  that  to  the  majority 
(and    the    word    majority    is  Fig.  6. 

used  with  intention)  of  people 
exactly  matches  the  colour 
of  the  bichromate.  Thus  we 
have  a  proof  that,  as  far  as 
the  eye  is  concerned,  the 
simple  and  the  complex 
colours  are  identical.  This  illustration  of  the  want  of 
power  of  the  eye  to  analyse  colour  might  be  repeated  as 
often  as  we  like.  We  may  pass  coloured  wools,  for  in- 
stance, through  the  length  of  the  spectrum  and  show 
that  they  have  the  property  of  appearing  bright  in, 
and  therefore  of  reflecting,  some  colours  and  of  almost 
disappearing  in  others — a  sure  indication  that  these 
colours  are  mixed  colours  as  they  are  made  up  of  the 
rays  which  are  reflected.  Yet  when  viewed  in  white 
light  they  can  in  many  cases  be  matched  with  simple 


24  Colour  Vision. 

colours  in  the  way  we  matched  the  colour  of  the 
bichromate  solution.  This  tells  us  that  there  is  some- 
thing which  requires  investigating  as  to  the  con- 
stitution of  the  perceiving  apparatus,  and  points  to 
the  probability  that  it  is  less  complicated  than  it  would 
be  were  it  able  to  differentiate,  without  the  aid  of  the 
spectrum,  between  simple  and  complex  colours.  If 
the  eye  had  a  separate  apparatus — and  when  I  say 
apparatus  I  use  the  word  for  want  of  a  better — for 
taking  up  the  impression  of  every  simple  colour,  it 
might  well  be  assumed  that  a  differentiation  must  take 
place. 

There  is  one  class  of  colours,  it  must  be  remembered, 
which  can  never  be  mistaken  for  simple  colours.  I  refer 
to  the  purples — mixtures  of  red  and  blue — for  there  are 
no  spectrum  colours  which  unmixed  can  possibly  match 
them.  All  other  colours,  as  no  doubt  will  soon  be 
apparent,  can  be  referred  to  some  one  spectrum  colour, 
either  in  its  pure  state  or  else  mixed  with  some 
variable  quantity  of  white  light.  We  are  all  familiar 
with  the  fact  that  there  are  three  primary  colours,  and 
we  are  naturally  led  to  consider  these  in  the  light  of 
the  experiments  just  made.  As  good  a  definition  as 
any  other  of  a  primary  colour  is  that  it  is  a  colour  which 
cannot  be  formed  by  the  mixture  of  any  two  or  more 


Primary  Colours,  :  5 

colours.  The  original  investigators  in  colour  phenomena 
were  the  artists,  and  they  found  that  neither  red,  nor 
yellow,  nor  blue  could  be  formed  by  any  mixture  of 
pigments  on  their  palette,  but  that  all  other  colours 
could  be  made  by  a  mixture  of  two  or  more  of  these 
three.  Hence  to  these  three  were  given  the  name  of 
primary  colours.  When,  however,  the  physicist  began 
to  work  with  the  simple  colours  of  the  spectrum,  it 
was  speedily  found  that,  at  all  events,  the  yellow  was 
not  a  primary  colour,  as  it  could  be  formed  by  a 
mixture  of  green  and  red,  whilst  a  green  could  not 
be  formed  by  a  mixture  of  any  other  two  colours. 
This  we  can  prove  with  our  apparatus. 

Three  apertures,  all  of  which  can  be  opened  or  closed 
as  required  (see  Fig.  7),  are  placed 
in  the  spectrum,  one  in  the  red,  one 
in  the  green,  and  one  in  the  violet. 
The  last  we  shall  not  require  at 
present,  so  it  is  entirely  closed ;  but 
we  vary  the  width  of  the  other  two. 
We  find  that  with  a  little  red  added 
to  a  bright  green^  a  yellow  green  is 
produced ;  wdth  more  red  added  we 
have  yellow ;  with  still  more  red,  an 
orange.     The  relative  brightness  of 


Fig. 


26  Colour   Vision. 

the  two  colours  mixed  together  can  be  shown  by  re- 
moving the  lens  which  recombines  the  spectrum  to  form 
the  patch  of  light.  Each  colour  issues  through  its  slit 
and  forms  its  own  patch  on  a  white  screen  which,  for 
the  purpose,  we  make  rather  larger 
than  usual.  The  two  patches  overlap 
in  the  middle  (Fig.  8),  and  the  pure 
colours  are  seen  one  on  each  side  of 
the  mixed  colours. 

Now%  placing  one  slit  in  the  yellow 
and  another  in  the  blue  of  the  spectrum,  we  find  that 
whatever  width  of  slit  we  take,  no  green  is  produced, 
but  that,  in  fact,  a  yellowish  or  a  bluish  white  results, 
and  that  when  the  two  slits  are  properly  adjusted,  a 
pure  white  is  produced.  Evidently  since  none  of  the 
intermediate  spectrum  colours  between  the  blue  and  the 
yellow  can  be  made  by  their  mixture,  certainly  green 
cannot.  Hence,  with  pure  colours  a  green  and  not  a 
yellow  is  one  of  the  primaries. 

Further  investigation  on  these  lines  has  placed  the 
violet  of  the  spectrum  as  a  primary  rather  than  the  blue, 
but  this  is  still  a  matter  of  debate.  Suffice  it  to  say 
that  a  red  and  a  green  in  the  spectrum  are  really  two  of 
the  primary  colours,  and  most  probably  the  violet  the 
third.       Experiment    shows    that    there    is   no    other 


^^^V  Pigments.  %^^^m^^. 

^primary   colour  in  the  strict  sense ^(^i^^^«»id^T^e '  ^ 
thus  arrive  at  the  fact  that,  except  tll^^^^I]^|f^o^^^^^^^^  1 
themselves,   every  colour  in  nature  may  'lie  ■  piad^xby  ^tj 
a  mixture  of  two  or  three  of  these  primaries.  **  -'^-...--*^ 
Just  a  word  of  explanation  as  to  why,  with  pigments, 
the    primary    colours    appear    to    be    red,   yellow,  and 
blue,  and  not  red,   green,   and  blue.     The  colour  of  a 
pigment,  it  must  be  recollected,  is  a  complex  one.     If  we 
analyse  a  yellow — a  yellow  glass  will  be  just  as  good 
an  example  as   anything  else — w^e   find   it  is  m.ade  up 
of  green,   yellow,  orange,  and  red.      A  blue  is  made 
up  of  blue  and  green.     If  a  yellow  is  placed  behind 
a  blue  glass,  and  we  look   at  a  white  surface  through 
them,  the  only   light  that  can  get  through  the  glass 
is  the  green.     If  the  light,  coming  through  each  glass 
separately,  falls  on  the  same  spot  on  a  white  surface,  it 
will  be  either  colourless  or  bluish  white,  or  yellowish 
white,   whichever  colour   preponderates.     As  the  light 

I  reflected  from  mixed  pigments  is  made  up  principally 
by  the  light  coming  through  the  different  particles, 
first  coming  through   one  and  then   through   another, 

1  and  only  partially  by  mixed  lights,  it  will  be  gathered 
why  the   primary   colour,  when  deduced  from  experi- 
ments with  pigments,  was  yellow,  and  not  green. 
With    the    spectrum    colours    there    is    this   fact    to 


28  Colour   Vision. 

remember,  that  though  all  intermediate  colours  between 
the  pairs  of  primaries  can  be  formed  by  their  mixture, 
yet  in  some  cases  the  resulting  colours  are  slightly 
diluted  with  white,  and  that  they  thus  appear  less 
saturated  than  the  spectrum  colours  themselves.  The 
reason  for  this  we  shall  be  able  to  account  for  when 
we  consider  the  colour  sensations  themselves. 

When  making  matches  to  simple  or  other  colours  l)y 
the  method  of  mixtures,  we  have  to  be  careful  of  the 
conditions  under  which  we  experiment.  This  can  be 
shown  by  a  very  simple  experiment.  I  will  make  a 
match  on  B  with  the  white  light,  which  is  thrown  on 
the  surface  A  (Fig.  6),  by  mixing  the  red,  green,  and 
violet  that  pass  through  the  three  adjustable  apertures 
or  slits  already  described.  The  apertures  are  altered  till 
the  match  appears  to  myself  perfect.  From  an  appeal 
made  to  those  of  the  audience  who  are  at  least  25  feet 
away  from  the  patches  of  light,  as  to  the  correctness  of 
the  match,  I  gather  that  the  match  is  to  them  imperfect. 
The  mixed  colours  appear  to  them  to  give  a  pinkish 
white.  The  reason  of  this  defect  in  the  match  is  due  to 
the  fact  that,  as  the  lecturer  is  viewing  the  two  square 
patches  of  2  in.  side  from  a  distance  of  2  ft.  6  in.,  their 
images  on  his  retina  extend  beyond  the  boundary  of 
the  yellow  spot,  whilst  the  audience  receives  the  whole 


Colour  Matches  and  the    Yellow  Spot.  29 

of  the  image  on   that  portion   of  the   retina   which  is 
completely  covered  by  it.     To  the  lecturer  only  part  of 
the  blue  and  green  is  absorbed  by  the  yellow  spot,  and 
the  part  of  the  retina  outside  it  on  which   the  image 
falls  receives  and  records    the    full   intensity   of  these^ 
colours.     To  the  audience  the  full  amount  of  absorption 
takes  place,  with  the  result  that  the   patch   of  mixed 
colours  must  appear  too  red  when  it  is  correct  to  the 
lecturer.     In   this   case  habit  makes   the   eye   take   an 
average  of  the  different  intensities  which  must  exist  at 
the  various  parts  of  the  image.     We  can,  however,  cause 
I  perfect  agreement  between  all  parties  if  the  experi- 
menter views  the  surfaces  in  a  mirror  placed  some  12 
^eet  away  and  then  makes  the  match,  for  he  is  viewing 
]he  patches  from  what  is  practically  a  distance  of  24 
eet.     If  after  making  the  match  without  the  aid  of  the 
nirror  the  lecturer's  eyes  are  directed   a  little  to  one 
ide  of  the  illuminated  surfaces,  a  match  will  no  longer 
xist ;    the    mixed    colour,   which    is    to    the    audience 
)inkish,  will  now  appear  a  bluish  green  to  him.     The 
eason  for  this  alteration  in  hue  is  that  the  whole  of  the 
mages  falls  outside  the  yellow  spot.  t 

It  will  now  be  quite  apparent  that  we  must  discount 
ny  assertion  in  regard  to  colour  matches,  unless  we  are 
old  the  distance  of  the  eye  from  the  surface  on  which 


30  Colour   Vision. 

the  match  is  made,  together  with  the  size  of  that  sur- 
face. This  yellow  spot  is  often  provokingly  tiresome 
in  the  study  of  colour  mixtures,  and  one  might  almost 
be  justified  in  doubting  whether  any  absolutely  exact 
matches  can  ever  be  vouched  for,  owing  to  the  important 
region  of  the  retina  which  it  occupies. 

The  fatigue  of  the  retina  to  colour  after  it  has 
been  presented  to  the  eye  for  any  length  of  time  is  a 
•difficulty,  but  in  a  less  degree.  That  the  retina 
does  experience  fatigue  can  be  shown  by  a  very  simple 
experiment.  The  lecture  theatre  is  now  illuminated 
by  the  incandescent  light,  and  if  we  throw  an  image 
of  the  bright  carbon  points  of  the  electric  arc  light 
•on  the  screen  and  steadily  fix  the  eyes  on  the  image 
of  the  white-hot  crater  for  some  (say)  tw^enty  seconds, 
and  then  we  suddenly  withdraw  it,  a  dark  image  of  the 
points  will  be  seen  on  the  partially  lighted  screen,  and 
will  appear  to  travel  with  the  eyes  as  they  move  away 
from  the  fixed  point.  This  phenomenon  is  due  to  the 
fact  that  the  perceiving  apparatus  for  white  light  gets 
fatigued  on  the  parts  of  the  retina  on  which  the  bright 
image  of  the  white  carbon  points  thrown  on  the  screen 
fell,  and  that  when  the  source  of  brightness  was  removed, 
the  less  intense  illumination  of  the  screen  failed  to 
.stimulate  the  vision  apparatus  at  those  parts  to  the 


Retinal  Fatigue,  31 

same  extent  that  they  were  stimulated  over  the  rest  of 
the  field.  We  can  vary  the  experiment  by  placing  a  red 
glass  in  front  of  the  electric  light,  and,  following  the 
same  course  as  before,  we  shall  see  a  greenish-blue 
image  of  the  carbon  points  upon  the  screen.  In  this 
case  the  retinal  apparatus  w^hich  has  not  been  stimu- 
lated by  the  red  sensation  will  be  capable  of  the 
maximum  stimulation  by  the  feeble  white  light,  whilst 
that  part  which  has  suffered  fatigue  will  not  respond  so 
freely  to  the  red  contained  in  the  white  light.  If  we 
abstract  a  certain  amount  of  red  from  the  spectrum, 
its  recombination  will  give  a  white  tinged  with  greenish 
blue,  which  is  a  counterpart  of  the  colour  we  feel  when 
.the  eyes  have  been  fatigued  by  the  red  light. 


(32    ) 


CHAPTEE     III. 

Let  mc  take  you  back  again  to  matches  of  colour. 
"We  will  now,  however,  make  the  matches  with 
the  primary  colours  in  the  guise  of  pigments.  These 
colours  themselves  are  complex  colours,  but  as  the 
eye  cannot  trace  any  difference,  or  at  all  events  very 
little  difference,  between  them  and  simple  colours, 
a  mixture  of  these  complex  colours  should  answer 
nearly  as  well  as  do  mixtures  of  the  simpler  colours. 
We  have  here  three  discs,  a  red,  a  green,  and  a  blue, 
and  we  can  very  closely  match  these  colours  by  a 
red,  a  green,  and  a  blue  in  the  spectrum. 

By  having  a  radial  slit  cut  to  the  centre  of  these  card 
discs,  we  can  slip  one  over  the  other  so  as  to  expose 
all  three  colours  as  sectors  of  a  single  disc.  Then  we 
can  place  the  compounded  disc  on  the  axis  of  a  rapidly 
rotating  motor,  and  the  colours  will  blend  together, 
giving  an  uniform  colour.  Any  proportions  of  the 
three  colours  can   thus   be   mixed,   and  by  a  judicious 


Colour  Discs.  33 

alteration  in  them  we  now  have  them  so  arranged 
that  they  give  a  grey.  By  inter-locking  together 
(Fig.  9)  a  black  disc  and  a  white  disc,  each  with  a 
diameter  slightly  larger  than  that  of  the  other  discs, 
but  equal  to  each  other,  and  rotating  them  on  the 
same  spindle  behind  the  three  colour  discs,  we  can, 
by  an  alteration  in  the  proportion  of  black  to  white, 
form   a    grey   which    will    match    that  Fig.  9. 

produced  by  the  rotation  of  the  three 
coloured  sectors.  In  other  words,  white, 
though  degraded  in  tone,  can  be  pro- 
duced by  the  three  complex  pigment 
colours,  as  we  have  seen  can  also  be  done  by  the 
mixture  of  the  three  simple  spectrum  colours. 

The  mixture  of  the  three  spectrum  colours  can 
match  other  colours  than  white.  For  instance,  it 
can  be  made  to  match  the  colour  of  brown  paper. 
By  the  colour  discs  also  we  can  do  exactly  the  same 
by  introducing,  if  necessary,  a  small  quantity  of  white 
or  black,  or  both,  to  dilute  the  colour  or  to  darken 
its  tone. 

Another  application  of  the  same  principles  enables  us 
to  produce  an  artificial  spectrum  by  means  of  a  red,  a 
green,  and  a  blue  glass.  By  fixing  these  three  glasses 
behind  properly  shaped  apertures  cut  in  a  card  disc  at 

D 


34  Colour  Vision, 

proper  radial  distances  from  the  centre,  and  rotating 
the  disc,  we  have  upon  the  screen  when  light  is  passed 
through  them  a  ring  of  rainbow  colours.  If  the 
beam  of  light  be  first  passed  through  a  suitable 
rectangular  aperture,  the  breadth  of  which  is  small 
compared  with  its  length,  placed  close  to  the  rotating 
disc,  and  an  image  of  the  aperture  be  focussed  on  the 
screen  by  a  suitable  lens,  we  shall  have  a  very  fair 
representation  of  the  spectrum — every  colour  inter- 
mediate between  the  red  and  green,  or  the  green 
and  blue,  being  formed  by  mixtures  of  these  pairs 
respectively. 

We  have  now  given  a  very  fair  proof  that  vision  is 
really  trichromic — that  is,  that  it  is  unnecessary  to 
have  more  than  the  sensations  of  three  colours  to 
produce  the  sensation  of  any  of  the  others. 

There  is  one  colour,  if  it  may  be  called  so,  that  has 
not  been  shown  you,  and  whether  it  is  a  simple  colour 
or  not  cannot  be  stated.  It  seems,  however,  to  be  the 
basis  of  all  other  colours,  since  they  all  commence  with 
it.  It  would,  perhaps,  be  preferable  to  call  it  the  first 
perception  of  light  instead  of  a  colour.  We  can  ex- 
hibit this  in  a  fairly  easy  manner  by  a  little  artifice. 
An  incandescent  lamp  is  before  you,  and  a  current 
from   a   battery   passing    through   the    carbon    thread 


Fig.  10. 


Fundamental  Light.  35 

causes  it  to  glow  brightly.  In  the  circuit,  however, 
I  have  introduced  what  is  known  as  a  resistance, 
which  consists  of  a  very  large  number  of  square  pieces 
of  carbonized  linen,  pressed  more  or  less  tightly  to- 
gether. By  means  of  a  screw  the  pressure  can  be 
varied.  When  the  pressure  is  somewhat  relaxed,  the 
resistance  to  the  passage  of  the  current  is  increased,  and 
the  carbon  thread  glows  less  brightly ;  and  by  a  still 
greater  release  of  pressure, 
the  light  can  be  made  to  dis- 
appear altogether.  A  beaker 
(Fig.  10)  which  we  have 
here  is  covered  with  thin 
blotting  paper,  and  when 
placed  over  the  incandescent 
glow-lamp  it  appears  as  a  luminous  yellow  cylinder,  the 
colour  being  due  to  that  of  the  light  within  it.  We 
can  next  insert  more  resistance  in  the  circuit,  and  it 
becomes  red,  due  to  the  ruddy  light  of  the  thread. 
By  inserting  still  more  resistance  into  the  circuit  the 

Ied  fades  away,  but  in  the  darkness  of  this  lecture 
heatre  the  beaker  is  still  a  luminous  object,  though 
iaintly  so.  It  has  no  colour,  and  the  only  sensation 
fc  provokes  is  one  of  light.  Taking  off  the  beaker,  we 
see  that  the  carbon  thread  is  a  dull  red  and  nothing 


D    2 


36  Colour  Vision, 

more.  The  passage  of  tliis  light  through  the  white 
blotting  paper  so  reduces  it  that  the  red  is  non- 
existent, and  the  initial  sensation  is  all  we  perceive. 

Placing  a  piece  of  red,  green,  or  blue  gelatine  round 
the  lamp,  we  get  the  same  effect,  showing  that  the 
basis  of  all  colour,  be  it  red,  green,  or  any  other  colour, 
is  what  appears  to  us  to  be  colourless.  This  experi- 
ment is  one  which  is  full  of  interest,  as  it  has  a  very 
distinct  bearing  on  diagnosing  our  colour  sensations, 
and  a  variation  of  it  will  have  to  be  repeated  under 
other  conditions. 

To  go  back,  however,  a  little  way,  how  does  it  arise 
that  only  three  sensations  are  necessary  to  give  the 
impression  of  all  colours  ?  One  can  understand  that 
some  definite  period  of  the  ether  waves  might  be  in 
unison  with  the  possible  swing  of  one  apparatus  in 
the  eye,  and  another  with  another,  but  it  is  some- 
what difficult  at  first  sight  to  conceive  that  more 
than  one  can  be  made  to  answer  to  wave  motion  of 
a  period  with  which  it  is  out  of  tune,  so  to  speak.  A 
couple  of  illustrations  taken  from  physical  experiments 
may  help  to  suggest  how  this  can  happen. 

Fig.  11  is  a  double  pendulum  arranged  as  shown. 
The  pendulum  A  is  heavily  weighted,  whilst  the  pen- 
dulum B  is  light,    being   only  a  string   with  a  small 


Penduhmi  Motion. 


37 


weight  attached.  This  difference  in  weight  was  made 
designedly,  to  prevent  any  great  effect  of  the  move- 
ment of  B  being  shown  on  A,  though  Fkj  h 
that  of  A  must  necessarily  exercise  a 
great  influence  on  B.  The  two  pendu- 
lums are  now  of  the  same  length.  A 
is  set  in  motion,  and  as  it  swings,  B 
also  begins  to  swing,  and  soon  is  os- 
cillating with  greater  motion  than  A, 
and  continues  to  do  so.  The  length  of  ^ 
the  pendulum  B  is  next  shortened,  and  A  is  again  s^t 
in  motion.  B  takes  up  the  motion,  and  increases  its 
swing  more  and  more,  but  now  the  two  pendulums 
are  in  opposite  phases,  and  the  motion  of  A  tends 
to  diminish  the  swing  of  B,  and  continues  to  do 
so  till,  after  an  interval  of  time,  B  is  once  more 
at  rest,  when  it  again  will  start  swinging.  The  fact  is, 
that  w^hen  A  commences  to  swing,  B  also  commences ; 

I  ad  as  long  as  B  and  A  are  moving  in  the  same  direc- 
ion  the  impulses  tend  to  make  B  increase  its  swing, 
ut  when  they  are  moving  in  the  opposite  direction,  or 
Either,  perhaps  it  should  be  said,  when  A  begins  to  start 
from  the  highest  point  of  its  swing  downwards  whilst 
B  is  travelling  upwards,  the  swing  of  -B  will  gradually 
diminish.     This,  of    course,  must   happen    when    B   is 


38  Colour  Vision, 

shorter  or  longer  tlian  A,  since  their  times  of  oscillation 
are  then  different.  We  can  now  picture  to  ourselves 
that  when  in  the  perceiving  apparatus  in  the  retina 
the  moving  parts — probably  molecules  or  atoms — 
arrive  at  a  certain  amplitude,  there  is  then  an  im- 
pression of  light,  and  that  it  is  quite  possible  that 
not  only  those  waves  whose  motion  is  exactly  of  the 
same  period  as  that  of  the  apparatus  will  set  them 
in  motion,  but  also  those  waves  which  are  actually 
of  a  very  different  period.  If  such  be  the  case,  it 
can  be  seen  that  waves  of  light  of  some  periods  may 
set  each  of  the  three  kinds  of  perceiving  apparatus  in 
motion,  and  that  possibly  the  resulting  impressions 
given  by  the  sum  of  all  three  for  a  Avave  out  of  tune 
with  any  of  them  may  be  even  greater  than  when  the 
wave  period  is  absolutely  the  same  as  one  of  them. 
For  in  the  last  case  a  maximum  effect  may  be  pro- 
duced on  one  apparatus,  and  the  effects  on  the  other 
two  may  be  insignificant ;  whilst  in  the  first  case  the 
effects  on  two  of  them  may  be  so  large  that  their 
combined  effects  may  have  a  larger  value. 

The  following  diagram  (Fig.  12),  made  on  the  prin- 
ciple of  Lissajou's  figures,  shows  graphically  the  motion 
of  the  pendulum.  The  pendulum,  with  a  pen  attached, 
was  started  by  an  independent  pendulum,  which  had  a 


Graphic  Periduhim  Motion.  39 

different  period,  and  the  amplitude  of  the  former 
registered  itself  on  paper  which  moved  by  clockwork 
round  the  axis  of  suspension.  As  the  two  pendulums 
had  different  periods,  the  amplitude,  as  shown  by  the 
traces  made,  first  increased  and  then  diminished  till 
there  was  no  motion,  and  then  started  again.  The  trace 
is  very  instructive,  and  deserves  attention.  It  will 
be  noticed  that  the  amplitude,  or  length  of  swing, 
increased  rapidly  at  first,  and  then  very  gradually 
attained  a  maximum.  ^^^-  ^2- 

Having  attained  this 
maximum,  the  ampli- 
tude diminiBhed  very 
slowly  for  some  time, 
and  finally  came  rather  rapidly  to  zero,  and  the 
pendulum  for  an  instant  was  at  rest. 

With  the  notion  in  our  minds  that  the  perceiving 
apparatus  might  act  in  the  way  that  the  pendulum 
acts,  we  naturally  apply  it  to  the  theories  which  early 
investigators  on  colour  vision  propounded.  Thomas 
Young,  whose  name  has  already  been  mentioned,  had 
propounded  a  theory  of  vision,  which  depended  on 
the  existence  of  only  three  colour  sensations,  and 
Von  Helmholtz  adopted  it  and  explained  the  action 
of  the  three  sensations  in  reference  to  the  spectrum  as 


40 


Colo7cr   Vision. 


shown  in  the  diagram.  These  figures  do  not  pretend 
to  be  absolute  measures  of  the  sensations,  but  only 
of  the  form  which  they  might  take  (Fig.  13).  The 
height  of  the  curve  at  each  part  of  the  spectrum  is 
supposed  to  represent  the  stimulation  given  to  each 
apparatus  by  the  different  colours.  Looking  at  the 
figures   we   see   that   each   sensation    has   a    place   of 

maximum  stimula- 

FiG.  13.    ^^.^^^^^^ 

tion,  and  that  the 
stimulation  falls 
off  more  or  less 
rapidly  on  each 
side  of  this  maxi- 
mum. It  will,  how- 
ever, be  noticed 
that  whilst  the  green  sensation  takes  very  much  the 
form  of  the  pendulum  amplitudes  (Fig.  12)  between  its 
periods  of  rest,  the  other  two  differ  from  it.  In  the  case 
of  the  red  sensation,  the  stimulation  falls  very  rapidly 
in  the  red  as  it  reaches  the  limit  of  visibility  of  the 
spectrum,  and  in  that  of  the  blue  sensation  the  steep 
descent  is  towards  the  extreme  violet.  When  the  three 
sensation  theory  is  examined  in  the  light  of  the  careful 
measurements  that  have  been  made,  the  results^  tell  us 
that  these  diagrams  can  only  be  taken  as  suggest^ 


V  Bl.  Gr.         Y         O  R 

The  top  figure  is  tlie  red  sensation  on  the  Young 
theory ;  the  middle  is  tlie  green  sensation,  and 
the  lowest  the  violet  or  bhie  sensation. 


(41   ) 


CHAPTER    IV. 

An  independent  investigator  of  this  subject  was 
Clerk  Maxwell,  who  experimented  with  a  "  colour-box  " 
of  his  own  design,  by  which  he  mixed  the  simple 
colours  of  the  spectrum,  and  the  results  he  got  are 
really  the  first  which  are  founded  on  measurement. 
He  measured  something,  but  liardly  arrived  at  the 
colour  sensation.  His  colour-box  took  two  forms, 
both  on  the  same  principles,  so  only  one  will  be  here 
described,  the  diagram  and  description  being  taken 
from  his  classic  paper  in  the  Philosophical  Transactions 
of  the  Poyal  Society  for  1860. 

"The  experimental  method  which  I  have  used 
consists  in  forming  a  combination  of  three  colours 
belonging  to  different  portions  of  the  spectrum,  the 
quantity  of  each  l)eing  so  adjusted  that  the  mixture 
shall  be  white,  and  equal  in  intensity  to  a  given  white* 
Fig.  14  represents  the  instrument  for  making  the 
observations.     It  consists  of  two  tubes,  or  long  boxes 


42  Colour  Vision. 

of  deal,  of  rectangular  section,  joined,  together  at  an 
angle  of  about  100°. 

"  The  part  A  K  is  about  five  feet  long,  seven  inches 
broad,  and  four  deep;  K  N  is  about  two  feet  long, 
^\^  inches  broad,  and  four  deep  ;  B  D  is  a  partition 
parallel  to  the  side  of  the  long  box.  The  whole  of 
the  inside  of  the  instrument  is  painted  black,  and  the 
only  openings  are  at  the  end  AC,  and  at  E.     At  the 

Fig.  14. 


c,_ 

Y< 


Maxwell's  colour-box. 

angle  there  is  a  lid,  which  is  opened  when 
the  optical  parts  have  to  be  adjusted  or 
cleaned. 

"  At  E  is  a  fine  vertical  slit,  L  is  a  lens  ;  at  P  there 
are  two  equilateral  prisms.  The  slit  E,  the  lens  L, 
and  the  prisms  P  are  so  adjusted,  that  when  light  is 
admitted  at  E,  a  pure  spectrum  is  formed  at  A  B,  the 
extremity  of  the  long  box.  A  mirror  at  M  is  also 
adjusted  so  as  to  reflect  the  light  from  E,  along  the 
narrow  compartment  of  the  box  to  B  C. 

"At  A  B  is  a  rectangular  frame  of  brass,  having  a 
rectangular  aperture  of  six  inches  by   one.     On    this 


MaxweWs  Colour-Box.  43 

frame  are  placed  six  brass  sliders,  X  Y  Z.  Each  of 
these  carries  a  knife-edge  of  brass  in  the  plane  of  the 
surface  of  the  frame. 

''  These  six  movable  knife-edges  form  three  slits, 
X  Y  Z,  which  may  be  so  adjusted  as  to  coincide  with 
any  three  portions  of  the  pure  spectrum  formed  by 
light  from  E.  The  intervals  behind  the  sliders  are 
closed  by  hinged  shutters,  which  allow  the  sliders  to 
move  without  letting  light  pass  between  them. 

"  The  inner  edge  of  the  brass  frame  is  graduated  to 
twentieths  of  an  inch,  so  that  the  position  of  any  slit 
can  be  read  off.  The  breadth  of  the  slit  is  ascertained 
by  means  of  a  wedge-shaped  piece  of  metal,  six  inches 
long,  and  tapering  to  a  point  from  a  width  of  half  an 
inch.  This  is  gently  inserted  into  each  slit,  and  the 
breadth  is  determined  by  the  distance  to  which  it 
enters,  the  divisions  on  the  wedge  corresponding  to 
the  200th  of  an  inch  difference  in  breadth,  so  that 
the  unit  of  breadth  is   '005  inch. 

"  Now  suppose  light  to  enter  at  E,  to  pass  through 
the  lens,  and  to  be  refracted  by  the  two  prisms  at  P, 
a  pure  spectrum,  showing  Fraunhofer's  lines,  is  formed 
at  AB,  but  only  that  part  is  allowed  to  pass  which 
falls  on  the  three  slits,  XYZ.  The  rest  is  stopped 
by  the  shutters.     Suppose  that  the  portion  falling  on 


44  Colour  Vision. 

X  belongs  to  the  red  part  of  tlie  spectrum ;  then,  of 
the  white  light  entering  at  E,  only  the  red  will  come 
through  the  slit  X.  If  we  were  to  admit  red  light 
at  X,  it  would  be  refracted  to  E,  by  the  principle  in 
optics  that  the  course  of  the  ray  may  be  reversed. 

"  If,  instead  of  red  light,  we  were  to  admit  white  light 
at  X,  still  only  red  light  would  come  to  E ;  for  all 
other  light  would  be  either  more  or  less  refracted,  and 
would  not  reach  the  slit  at  E.  Applying  the  eye  at 
the  slit  E,  we  should  see  the  prism  P  uniformly 
illuminated  with  red  light,  of  the  kind  corresponding 
to  the  part  of  the  spectrum  which  falls  on  the  slit  X, 
when  white  light  is  admitted  at  E. 

"  Let  the  slit  Y  correspond  to  another  portion  of  the 
spectrum,  say  the  green  ;  then  if  white  light  is  admitted 
at  Y,  the  prism,  as  seen  by  an  eye  at  E,  will  be  uni- 
formly illuminated  with  green  light ;  and  if  white 
light  be  admitted  at  X  and  Y  simultaneously,  the 
colour  seen  at  E  will  be  a  compound  of  red  and  green, 
the  proportions  depending  on  the  breadth  of  the  slits 
and  the  intensity  of  the  light  which  enters  them. 
The  third  slit  Z,  enables  us  to  combine  any  three 
kinds  of  light  in  any  given  proportions,  so  that  an 
eye  at  E  shall  see  the  face  of  the  prism  at  P, 
uniformly  illuminated  with  the  colour  resulting  from 


Maxwell's  Coloiir-Box.  45 

the  combination  of  the  three.  The  position  of  these 
three  rays  in  the  spectrum  is  found  by  admitting  the 
light  at  E,  and  comparing  the  position  of  the  slits 
with  the  position  of  the  principal  fixed  lines ;  and  the 
breadth  of  the  slits  is  determined  by  means  of  the 
wedges. 

"  At  the  same  time,  white  light  is  admitted  through 
BC  to  the  mirror  of  black  glass  at  M,  whence  it  is 
reflected  to  E,  past  the  edge  of  the  prism  at  P,  so 
that  the  eye  at  E  sees  through  the  lens  a  field  con- 
sisting of  two  portions,  separated  by  the  edge  of  the 
prism  ;  that  on  the  left  hand  being  compounded  of 
three  colours  of  the  spectrum  refracted  by  the  prism, 
while  that  on  the  right  hand  is  white  light  reflected  from 
the  mirror.  By  adjusting  the  slits  properly,  these  two 
portions  of  the  field  may  be  made  equal,  both  in  colour 
and  brightness,  so  that  the  edge  of  the  prism  becomes 
almost  invisible. 

"  In  making  experiments,  the  instrument  was  placed 
on  a  table  in  a  room  moderately  lighted,  with  the  end 
A  B  turned  towards  a  large  board  covered  with  white 
paper,  and  placed  in  the  open  air,  so  as  to  be  uniformly 
illuminated  by  the  sun.  In  this  way  the  three  slits 
and  the  mirror  M  were  all  illuminated  with  white 
light  of  the  same  intensity,  and  all  were  afiected  in  the 


46  Colour  Vision, 

same  ratio  by  any  change  of  illumination  ;  so  that  if 
the  two  halves  of  the  field  were  rendered  equal  when 
the  sun  was  under  a  cloud,  they  were  found  nearly 
correct  when  the  sun  again  appeared.  No  experiments, 
however,  were  considered  good  unless  the  sun  remained 
uniformly  bright  during  the  whole  series  of  experi- 
ments. 

"After  each  set  of  experiments  light  was  admitted 
at  E,  and  the  position  of  the  fixed  lines  D  and  F  of 
the  spectrum  was  read  oiff  on  the  scale  at  AB.  It 
was  found  that  after  the  instrument  had  been  in  use 
some  time  these  positions  were  invariable,  showing  that 
the  eye-hole,  the  prisms,  and  the  scale  might  be  con- 
sidered as  rigidly  connected." 

With  this  instrument  he  made  mixtures  of  three 
colours,  to  match  with  white.  By  shifting  the  slits 
into  various  positions  and  taking  as  his  three  standard 
colours  a  red  near  the  C  line,  a  green  near  E,  and  a 
blue  between  F  and  G-  (see  frontispiece),  he  obtained  a 
variety  of  matches,  from  which  he  formed  equations. 
After  eliminating,  or  rather  reducing  the  errors  to  the 
most  probable  value  by  the  method  of  least  squares,  he 
got  from  his  matches  with  white  a  table  of  colour  values 
in  terms  of  the  three  standard  colours,  from  which  the 
diagram  of  the  spectrum  (Fig.   15)  was  made.     (The 


Maxwell's  Colour  Curves. 


47 


heiahts  of  the  dotted  curves  are  derived  from  the 
widths  of  the  slits,  and  the  continuous  curve  is  the 
sum  of  these  heights.)  Now  what  appears  to  be  a 
properly  chosen  colour  does  not  necessarily  stimulate 
only  one  sensation.  Indeed  the  probabilities  are  against 
it,  except  in  the  extreme  red  and  extreme  violet.     If 

Fig.  15. 


colours  intermediate  to  the  standard  colours  be 
matched  by  a  mixture  of  the  latter,  we  do  not  arrive 
at  any  solution  of  the  amount  of  stimulation  of  each 

Insation,  since  the  chosen  standard  colours  themselves 
ay  be  due  to  a  stimulation  of  all  three  sensations, 
s  a  matter  of  fact.  Clerk  Maxwell  chose  colours 
hich  do  not  best  represent  the  colour  sensations. 
The  red  is  too  near  the  yellow,  as  is  also  the  green* 


48  Colour   Vision. 

The  blue  should  also  be  nearer  the  violet  end  of  the 
spectrum  than  the  position  which  lie  chose  for  it.  AVe 
may  take  it,  then,  that  except  as  a  first  approximation, 
Olerk  Maxwell's  diagrams  need  not  be  seriously  taken 
into  account.  The  diagram  itself  shows  that  the 
colour  sensations  are  not  represented  by  the  colours 
he  chose.  Supposing  any  one  in  whom  the  sensation 
of  green  is  absent  were  examining  the  spectrum,  there 
would,  according  to  the  diagram,  be  no  light  visible 
at  the  green  at  E.  Anticipating  for  a  moment  what 
we  shall  deal  with  in  detail  shortly,  it  may  be  stated 
that  in  cases  where  it  is  proved  that  a  green  sen- 
sation is  absent,  there  is  no  position  in  any  part  of 
the  spectrum  where  there  is  an  absence  of  light.  Had  -J 
he  chosen  any  other  green,  the  same  criticism  would 
have  been  valid.  The  diagram  as  it  stands  is  really  a 
diagram  of  colour  mixtures  in  terms  of  three  arbitrarily 
chosen  colours,  and  not  of  colour  sensations.  It  merely 
indicates  what  proportions  were  needed  of  the  three 
colours,  which  he  took  as  standards,  to  match  the  in- 
termediate spectrum  colours.  The  negative  sign  in 
3ome  of  the  equations — given  in  the  appendix,  page  201 
— may  be  somewhat  puzzling  to  those  who  have  not 
made  colour  matches,  but  not  to  those  who  have 
.actually  made   experiments.      It  means   that  where  it 


Young  s  Colour   Vision   Theory.  49 

is  present  no  matcli  of  colour  by  a  mixture  of  the 
standard  colours  is  possible;  and  that  it  would  be  only 
possible  if  a  certain  quantity  of  the  colour  to  which  is 
attached  a  negative  sign  were  to  be  abstracted — an 
impossible  condition  to  fulfil,  but  one  which  may 
often  occur  in  colour-matching  experiments.     Later  you 


will  find  that  when  colours  are  chosen  as  standards  so 
that  the  resulting  equations  give  no  negative  sign  for 
any  colour,  we  have  a  criterion  as  to  the  colours  which 
give  the  nearest  approach  to  the  true  sensations.  The 
next  diagram  (Fig.  16)  of  colour  sensations  is  due  to 
Koenig,  who  investigated  the  subject  with  Von  Helm- 
holtz.  By  a  modified  method,  which  perhaps  need  not 
be  explained  in  detail  here,  he  produced  them,  and 
they   must    be    apparently    not    far    from    the    actual 

E 


50  Colour   Vision, 

state  of  things,  supposing  this  theory  be  proved 
to  be  true.  For  my  own  part,  I  am  under  the 
impression  that  the  positions  of  the  colours  which 
most .  nearly  approach  the  colour  sensations  might  be 
slightly  altered  in  regard  to  the  green  and  the  blue, 
for  reasons  that  will  subsequently  be  given  when  the 
later  experiments  of  General  Testing  and  myself  come 
to  be  described.  For  the  immediate  purpose  of  the 
lecture,  the  curves  are  sufficiently  accurate,  and  I 
will  ask  you  to  notice  what  they  tell  us.  It  is 
presupposed  in  these  diagrams  that,  if  the  three 
colour  -  perceiving  apparatus  are  equally  stimulated, 
a  sensation  of  white  will  be  produced ;  and  the 
reverse,  of  course,  is  true,  in  that  white  will  give 
rise  to  equal  stimulation  of  the  three  apparatus.  It 
follows,  then,  that  in  the  parts  of  the  spectrum 
where  all  three  curves  of  sensation  are  seen  to  take 
a  part  in  the  production  of  a  colour,  such  as  at  the 
E  line,  the  colour  is  really  due  to  the  extra  stimu- 
lation of  one  or  two  of  the  apparatus  above  that 
required  to  produce  a  certain  amount  of  white.  The 
colour  in  every  part  of  the  spectrum  may  he  represented 
hy  not  more  than  two  sensations^  with  a  proportion  of 
^-  .  r  'liphiM>,^^ln  the  orange  and  scarlet  there  are  only  two 
^^'*^<?$^'^\^^"''/'^:^^^^%S^^-^^^*^^'    without    any   sensible   amount   of 


The  Three  Colour  Sensations.  51 

white,  as  the  amount  of  violet  sensation  is  extremely 
small.  At  the  extreme  ends  of  the  spectrum  only  one 
sensation — the  red  or  the  violet— is  excited ;  but  in  the 
region  of  the  green  the  colour  must  be  largely  diluted 
with  the  sensation  of  white.  As  an  example,  we  may 
take  the  part  of  the  spectrum  where  the  red  and  the 
violet  sensation  curves  cut  each  other.  At  this  point 
the  green  sensation  curve  rises  higher  than  the  inter- 
section of  the  other  curves.  The  red  and  the  violet 
sensations  have  only  to  be  mixed  with  an  equal  amount 
of  the  green  sensation  to  make  white,  so  that  the  height 
of  the  green  sensation  curve  above  the  point  of  inter- 
section represents  the  amount  of  pure  green  sensation 
which  is  stimulated.  The  colour  is  therefore  caused  by 
the  green  sensation,  largely  diluted  with  white.  A 
scrutiny  of  the  curves  will  show  that  at  no  point  is 
the  green  sensation  so  free  from  any  other  as  at  this 
point,  if  we  regard  white  by  itself  as  a  neutral  colour. 
Looking  at  these  figures,  we  can  readily  see  w^hat  effect 
the  removal  of  any  one  or  two  of  the  three  Sensations 
would  have  upon  the  colour  vision  of  the  individual. 
The  probabilities,  however,  against  two  of  the  three 
sensations  being  absent  must  evidently  be  very  much 
smaller  than  that  there  should  be  an  absence  of  only 
one  of  the  sensations,  either  red,  green,  or  violet. 

E    2 


52  Colour  Vision. 

It  will  be  well  that  we  should  also  have  1;)efore  us 
the  theory  which  is  the  only  serious  rival  to  that 
of  Young,  viz.,  that  of  Hering.  In  the  report  of 
the  Colour  Vision  Committee  there  is  an  excellent 
description  of  this  theory.  As  it  was  furnished  by 
Dr.  Michael  Foster,  we  may  be  sure  that  the  ideas 
of  its  originator  are  correctly  given,  and  therefore  I 
will  quote  it  in  his  words  : — 

*'  Another  theory,  that  of  Hering,  starts  from  the 
observation  that  when  we  examine  our  own  sensations 
of  light  we  find  that  certain  of  these  seem  to  be  quite 
distinct  in  nature  from  each  other,  so  that  each  is 
something  sui  generis^  whereas  we  easily  recognise  all 
other  colour  sensations  as  various  mixtures  of  these. 
Thus,  the  sensation  of  red  and  the  sensation  of  yellow 
are  to  us  quite  distinct ;  we  do  not  recognise  anything 
common  to  the  two,  but  orange  is  obviously  a  mixture 
of  red  and  yellow.  Green  and  blue  are  equally  distinct 
from  each  other  and  from  red  and  yellow,  but  in  violet 
and  purple  we  recognise  a  mixture  of  red  and  blue. 
White  again  is  quite  distinct  from  all  the  colours  in  the 
narrower  sense  of  that  word,  and  black,  which  we  must 
accept  as  a  sensation,  as  an  affection  of  consciousness, 
even  if  we  regard  it  as  the  absence  of  sensation  from  the 
field  of  vision,  is  again  distinct  from  everything  else. 


Herings  Colour   Vision  Theory.  53 

Hence  the  sensations  caused  by  different  kinds  of  light 
or  by  the  absence  of  light,  which  thus  appear  to  us 
quite  distinct,  and  which  we  may  speak  of  as  '  native ' 
or  '  fundamental '  sensations,  are  white,  black,  red, 
yellow,  green,  blue.  Each  of  these  seems  to  us  to  have 
nothing  in  common  with  any  of  the  others,  whereas  in 
all  other  colours  we  can  recognise  a  mixture  of  two  or 
more  of  these.  This  result  of  common  experience 
suggests  the  idea  that  these  fundamental  sensations 
are  the  primary  sensations,  concerning  which  we  are 
enquiring.  And  Hering's  theory  attempts  to  reconcile, 
in  some  such  way  as  follows,  the  various  facts  of 
colour  vision  Avith  the  supposition  that  we  possess 
these  six  fundamental  sensations.  The  six  sensations 
readily  fall  into  three  pairs,  the  members  of  each  pair 
having  analogous  relations  to  each  other.  In  each  pair 
the  one  colour  is  complementary  to  the  other — white  to 
black,  red  to  green,  and  yellow  to  blue.  Now,  in  the 
chemical  changes  undergone  by  living  subjects,  we  may 
recognise  two  main  phases,  an  upward  constructive 
phase,  in  which  matter  previously  not  living  becomes 
living,  and  a  downward  destructive  phase,  in  which 
living  matter  breaks  down  into  dead  or  less  living 
matter.  Adopting  this  view,  we  may,  on  the  one  hand, 
suppose   that  rays    of  light,   differing    in    their  wave- 


54  Colo2ir   Vision. 

length,  may  affect  the  chemical  changes  of  the  visual 
substance  in  different  ways,  some  promoting  construc- 
tive changes  (changes  of  assimilation),  others  promoting 
destructive  changes  (changes  of  dissimilation) ;  and  on 
the  other  hand,  that  the  different  changes  in  the  visual 
substance  may  give  rise  to  different  sensations. 

"  We  may,  for  instance,  suppose  that  there  exists  in 
the  retina  a  visual  substance  of  such  a  kind  that  when 
rays  of  light  of  certain  wave-lengths — the  longer  ones, 
for  instance,  of  the  red  side  of  the  spectrum — fall  upon 
it,  dissimilative  changes  are  induced  or  encouraged, 
while  assimilative  changes  are  similarly  promoted  by 
the  incidence  of  rays  of  other  wave-lengths,  the  shorter 
ones  of  the  blue  side.  But  it  must  be  remembered 
that  in  dealing  with  sensations  it  is  difficult  to 
determine  what  part  of  the  apparatus  causes  them  ; 
we  may  accordingly  extend  the  above  view  to  the 
whole  visual  apparatus,  central  as  well  as  peripheral, 
and  suppose  that  when  rays  of  a  certain  wave-length 
fall  upon  the  retina,  they  in  some  way  or  other,  in 
some  part  or  other  of  the  visual  apparatus,  induce  or 
promote  dissimilative  changes,  and  so  give  rise  to  sen- 
sations of  a  certain  kind,  while  rays  of  another  wave- 
length similarly  induce  or  promote  assimilative  changes, 
and  so  give  rise  to  a  sensation  of  a  different  kind. 


The  Six  Ftindamental  Sensations, 


"  The  hypothesis  of  Hering  applies  this 
six  fundamental  sensations  spoken  of  above,  an< 
supposes  that  each  of  the  three  pairs  is  the  outcome 
of  a  particular  set  of  dissimilative  and  assimilative 
changes.  It  supposes  the  existence  of  what  we  may 
call  a  red-green  visual  substance  of  such  a  nature  that 
so  long  as  dissimilative  and  assimilative  changes  are  in 
equilibrium,  we  experience  no  sensation ;  but  when 
dissimilative  changes  are  increased,  we  experience  a 
sensation  of  (fundamental)  red,  and  when  assimilative 
changes  are  increased,  we  experience  a  sensation  of 
(fundamental)  green. 

"  A  similar  yellow-blue  visual  substance  is  supposed 
to  furnish,  through  dissimilative  changes  a  yellow, 
through  assimilative  changes  a  blue  sensation  ;  and  a 
white-black  visual  substance  similarly  provides  for  a 
dissimilative  sensation  of  white '  and  an  assimilative 
sensation  of  black.  The  two  members  of  each  pair  are 
therefore  not  only  complementary  but  also  antagonistic. 
Further,  these  substances  are  supposed  to  be  of  such  a 
kind  that  while  the  white-black  substance  is  influenced 
in  the  same  way,  though  in  different  degrees,  by  rays 
along  the  whole  range  of  the  spectrum,  the  two  other 
substances  are  differently  influenced  by  rays  of  different 
wave-length.     Thus,  in  the  part  of  the  spectrum  which 


56 


Colour  Vision. 


we  call  red,  rays  promote  great  dissimilative  changes  of 
the  red-green  substance  with  comparatively  slight  effect 
on  the  yellow-blue  substance  ;  hence  our  sensation  of 
red. 

"In  that  part  of  the  spectrum  which  we  call  yellow, 
the  rays  eftect  great  dissimilative  changes  of  the  yellow- 

FiG.  17. 


B  G  Y  O  R 

blue  substance ;  but  their  action  on  the  red-green 
substance  does  not  lead  to  an  excess  of  either  dis- 
similation or  assimilation,  this  substance  being  neutral 
to  them ;  hence  our  sensation  of  yellow.  The  green 
rays,  again,  promote  assimilation  of  the  red- green 
substance,  leaving  the  assimilation  of  the  yellow-blue 
substance  equal  to  its  dissimilation  ;  and  similarly  blue 
rays  cause  assimilation  of  the  yellow-blue  substance, 
and  leave  the  red-green  substance  neutral. ,    Finally,  at 


Results  of  Disswiilation  and  Assimilation.       57 

the  extreme  blue  end  of  the  spectrum,  the  rays  once 
more  provoke  dissimilation  of  the  red-green  substance, 
and  by  adding  red  to  blue  give  violet.  When  orange 
rays  fall  on  the  retina,  there  is  an  excess  of  dissimilation 
of  both  the  red-green  and  the  yellow-ljlue  substance  ; 
when  greenish-blue  rays  are  perceived,  there  is  an 
excess  of  assimilation  of  both  these  substances  ;  and 
other  intermediate  hues  correspond  to  various  degrees 
of  dissimilation  or  assimilation  of  the  several  visual 
substances.  When  all  the  rays  together  fall  upon  the 
retina,  the  red- green  and  yellow-blue  substances  remain 
in  equilibrium,  but  the  white-black  substance  undergoes 
great  changes  of  dissimilation,  and  we  say  the  light  is 
white." 

It  has  been  said  by  the  same  writer  that  this  theory 
is  tri-chromic.  For  my  own  part  I  do  not  see  that 
it  is  so  in  the  sense  in  which  that  word  is  used  in 
the  theory  of  Young.  It  may  be  a  tetra-chromic,  for 
as  far  as  colour  is  concerned  the  black-white  sensation 
must  be  excluded  ;  but  it  appears  to  me  that  it  cannot 
be  strictly  brought  under  the  head  of  tri-chromic. 


(  58  ) 


CHAPTER    V. 

The  readiest  means  of  investigating  tlie  stimulation 
of  the  different  sensations  necessary  to  produce  colour 
is  evidently  by  eyes  in  which  one  or  two  sensations 
are  absent,  and  this  applies  not  only  to  the  Young 
theory,  but  also  to  that  of  Hering. 

In  former  days,  not  much  more  than  a  century  ago, 
the  existence  of  colour  blindness,  as  it  is  now  named^ 
was  a  matter  of  great  curiosity,  and  in  the  Philo- 
sophical Transactions  of  the  Poyal  Society  of  1777,. 
the  case  of  a  shoemaker  named  Harris  is  described 
by  a  Mr.  Huddart,  who  travelled  all  the  way  from 
London  to  the  Midlands  on  purpose  to  see  if  all  the 
alleged  facts  regarding  the  patient  were  true.  Harris 
mistook  orange  for  green,  brown  he  called  black,  and 
he  was  unable  to  distinguish  between  red  fruits  and 
the  surrounding  green  leaves.  At  first,  colour  blind- 
ness was  called  Daltonism,  from  the  fact  that  the 
great    chemist   Dalton    suffered    from    it,  and  investi- 


Daltonism.  59 

gated  the  variation  which  he  found  existed  in  his 
vision  from  that  of  the  majority  of  his  fellow-creatures. 
It  was  in  1794  that  Dal  ton  described  his  own  case  of 
colour  blindness.  He  was  quite  unaware  of  his  defect 
till  1792,  when  he  was  convinced  of  its  existence 
from  his  observations  of  a  pink  geranium  by  candle- 
light. "  The  flower,"  he  says,  "  was  pink ;  but  it 
appeared  to  me  almost  an  exact  sky-blue  by  day.  In 
candle-light,  however,  it  was  astonishingly  changed, 
not  having  any  blue  in  it ;  but  being  what  I  call  a 
red  colour  which  forms  a  striking  contrast  to  blue." 
He  goes  on  to  remark  that  all  his  friends  except  his 
hrother  (mark  this  relationship),  said :  there  was  not 
any  striking  difference  in  the  two  colours  by  the  two 
lights.  He  then  investigated  his  case  by  the  solar 
spectrum,  and  became  convinced  that  instead  of  having 
the  normal  sensations,  he  only  had  two  or  at  most 
three.  These  were  yellow,  blue,  and  perhaps  purple. 
In  yellow,  he  included  the  red,  orange,  yellow,  and 
green  of  others,  and  his  blue  and  purple  coincided 
with  theirs.  He  says,  that  "  part  of  the  image  which 
others  call  red,  appears  to  me  little  more  than  a  shade 
or  defect  of  light ;  after  that,  the  orange,  yellow  and 
green  seem  one  colour,  which  descends  pretty  uniformly 
from   an   intense    and   a  rare  yellow,   making  what  I 


6o  Colour   Vision. 

should  call  different  shades  of  yellow.  The  difference 
between  the  green  part  and  the  blue  part  is  very 
striking  to  my  eye,  they  seem  to  be  strongly  con- 
trasted. That  between  the  blue  and  purple  much  less 
so.  The  purple  appears  to  be  blue  much  darkened 
and  condensed." 

Dalton  said  a  florid  complexion  looked  blackish-blue 
on  a  white  ground.  Blood  looked  like  bottle  green, 
grass  appeared  very  little  different  from  red.  A  laurel 
leaf  w^as  a  good  match  to  a  stick  of  sealing-wax. 
Colours  appeared  to  him  much  the  same  by  moonlight 
as  they  did  by  candle-light.  By  the  electric  light 
and  lightning,  they  appeared  as  in  day  light.  Some 
browns  he  called  red,  and  others  black. 

Mr.  Babbage,  in  Scientific  London  (1874),  gives  an 
account  of  Dalton's  presentation  at  Court. 

Firstly,  he  w^as  a  Quaker,  and  would  not  wear  a 
aword,  which  is  an  indispensable  appendage  to 
ordinary  Court-dress.  Secondly,  the  robe  of  a  Doctor 
of  Civil  Laws  was  known  to  be  objectionable  on  ac- 
count of  its  colour — scarlet,  being  one  forbidden  by 
the  Quakers.  Luckily,  it  was  recollected  that  Dalton 
was  affected  with  that  peculiar  colour  blindness  which 
bore  his  name,  and  that  as  cherries  and  the  leaves  of  a 
cherry-tree  were  to  him  of  the  same  colour,  the  scarlet 


Statistics  of  Colour  Blindness.  .61 

gown  would  present  no  extraordinary  appearance.  So 
perfect  evidence  was  the  colour  blindness,  tliat  the 
most  modest  and  simple  of  men,  after  having  received 
the  Doctor's  gown  at  Oxford,  actually  wore  it  for 
several  days  in  happy  unconsciousness  of  the  effect 
he  produced  in  the  street.  The  rest  of  the  description 
we  need  not  reproduce.  Both  the  above  cases  we  shall 
see  shortly  come  under  the  category  of  red-blindness 
in  the  Young  theory.  Eecent  investigations  tell  us 
that  such  colour  blindness  is  by  no  means  rare,  nor  can 
it  have  been  then.  Statistics,  derived  from  carefully 
carried  out  examinations  made  in  A^arious  parts  of  the 
w^orld  by  an  approved  method  of  testing,  show  that 
about  four  out  of  every  hundred  males  suffer  from 
some  deficiency  in  colour  perception,  but  that  so  far 
as  the  more  limited  statistics  regarding  them  are  to 
be  depended  upon,  only  about  four  out  of  every  1000 
women  suffer  in  the  same  manner. 

Colour  blindness  in  a  healthy  subject  is  usually 
hereditary,  and  is  always  congenital.  It  is  curious 
to  trace  back  in  some  instances  the  colour  blindness, 
where  it  is  to  be  found,  in  a  family.  It  often  happens 
that  colour  blindness — as  the  gout  is  said  to  do — 
skips  a  generation.  This  is  usually  traced  to  the 
fact    that     the    generation    skipped    is    through    the 


62  Colour   Vision. 

motlier.  Thus,  the  maternal  grandfather  may  be 
colour  blind,  as  may  be  the  grandsons,  but  the  mother 
will  very  frequently  have  perfectly  normal  vision  for 
colour.  On  the  other  hand,  the  paternal  grandfather 
may  have  defective  colour  perception,  and  this  may 
be  inherited  both  by  the  grandsons  and  the  father. 
The  remark  made  by  Dalton  regarding  his  brother's 
eyesight  points  to  the  fact  that  his  own  colour  blind- 
ness was  probably  hereditary.  Deaf  mutes,  Jews  and 
Quakers,  seem  to  be  more  liable  to  colour  blindness 
than  other  people,  statistics  giving  them  13*7,  4*9, 
and  5  *  9  as  the  percentages.  It  may  be  well  to  point 
out  that  the  deficiency  in  colour  perception  to  which 
we  are  alluding  is  totally  distinct  from  that  which  may 
arise  from  disease.  This  last  form  has  such  marked 
characteristics  of  its  own  that  it  can  at  once  be  dis- 
tingfuished  from  the  cone^enital  form. 

Of  the  four  per  cent,  of  males  who  suifer  from 
congenital  colour  deficiency  of  vision,  a  large  number 
are  not  totally  lacking  in  any  one  or  more  colour 
sensations.  Those  in  which  one  sensation,  on  the 
Young  theory,  is  entirely  missing  are  called  "  com- 
pletely red-,  green-,  or  violet-blind,"  whilst  those  in 
which  the  sensation  is  but  partially  deadened  are 
called  "partially  red-,  green-,  or  violet-blind."     When 


Types  of  Colour  Blindness.  63 

two  sensations  are  entirely  absent,  and  such  cases 
are  very  rare  indeed,  they  are  generally  said  to  have 
monochromatic  vision  ;  that  is,  every  colour  to  them 
is  the  same,  as  is  also  white,  the  only  distinction 
between  any  of  them  being  the  superior  brightness  of 
some  over  others.  The  best  illustration  of  this  form 
of  colour  vision  is  perhaps  to  say  that  the  retina  of 
such  people  have  the  same  characteristics  in  regard 
to  sensitiveness  as  has  a  photographic  plate,  the 
resulting  prints  in  black  and  white  representing  what 
it  sees  in  nature.  When  we  have  to  adopt  the 
terms  used  by  the  followers  of  Hering's  theory — 
the  theory  which  obtains  most  followers  amongst 
the  physiologists,  since  it  endeavours  to  explain 
colour  vision  in  a  physiological  way,  though  it  fails 
to  meet  all  the  requirements  of  the  physicist — we 
should  restrict  our  terms  to  red- green  and  yellow- 
blue  blindness,  still  perhaps  retaining  the  term  mono- 
chromatic vision  for  the  rare  cases  specified  above. 
As  we  must  employ  some  terms  to  express  our  mean- 
ing, we  shall  in  these  lectures  adopt  those  of  the 
Young  theory. 

Now  taking  a  red-blind  person  and  examining  him 
with  the  spectrum,  we  find  that  he  sees  no  light  at 
all  at  the  extreme  limit   of  our  red,  and    only   when 


64  Colour   Vision. 

lie  comes  to  the  part  wliere  the  red  lithium  line  marks 
a  certain  red  does  a  glimmer  commence  ;  he  then  sees 
what  he  may  call  dark-green,  or  he  may  call  dark- 
yellow.  When  questioned  about  what  to  us  are  greens 
he  also  calls  them  green  or  yellow,  some  being  bright, 
others  saturated  hues,  and  others  again  paler.  When 
he  gets  to  the  bluish-green  he  calls  it  grey,  and  will 
say  it  is  indistinguishable  from,  and  in  fact  will  match 
with,  a  white  degraded  in  tone.  From  this  point  he 
will  say  he  sees  blue,  near  F  pale-blue,  and  in  the  violet 
dark-1)lue.  Too  much  importance  must  not  be  attached 
to  the  nomenclature  adopted  by  the  colour  blind.  They 
have  to  take  the  names  of  the  colours  from  the  normal 
eyed.  Yellow  objects  are  generally  brighter  than  red, 
and  having  annexed  the  idea  that  what  to  them  is 
bright  red  is  called  yellow,  they  give  it  that  dis- 
tinguishing name.  His  limit  of  vision  at  the  violet 
end  will  be  the  same  as  the  majority  of  mankind, 
but  it  will  be  considerably  shortened  at  the  red  end. 
The  point  in  the  spectrum  which  he  calls  grey  is  an 
important  point,  and  corresponds  to  the  place  where 
the  violet  and  green  curves  cut  in  Fig.  16.  This 
point  can  be  very  accurately  determined  by  placing 
alongside  the  colour  patch  A  (Fig.  6)  the  white 
patch,  which  is  reduced  in  brightness  as  required   by 


spectrum  Examination  of  the  Colour  Blind.     65 

rotating  sectors.  As  the  slit  is  moved  along  the 
spectrum  it  will  eventually  reach  a  point  where  he  will 
say  both  patches  of  light  are  exactly  similar  in  hue. 
To  the  normal  eye  one  will  be  white,  and  the  other 
the  kind  of  green  indicated  above  (see  frontispiece). 

If  a  similar  examination  be  made  of  the  green- 
blind,  the  red  end  of  the  spectrum  will  be  called 
red  or  yellow,  but  the  spectrum  itself  will  be  visible 
between  the  same  limits  as  it  is  to  the  person  who  has 
the  normal  sense  of  vision.  A  grey  stripe  will  be  seen 
in  the  spectrum,  but  in  this  case  it  will  be  a  trifle 
nearer  the  red  end  of  the  spectrum  than  the  point 
which  the  red-blind  calls  grey ;  from  this  point  to 
the  extreme  violet,  the  green-blind  will  name  the 
spectrum  colours  similarly  to  the  red-blind.  The 
part  of  the  spectrum  where  grey  exists  to  the  green- 
blind  is  even  more  important  than  that  part  at 
which  it  exists  to  red-blind,  for  it  marks  the  place 
where  the  red  and  violet  curves  cut  each  other  in 
Fig.  16,  and  is  in  the  majority  of  cases  the  place 
in  the  spectrum  where  to  the  normal  eye  the  green 
sensation  is  unmixed  with  any  sensation  except  that 
of  white,  as  quite  recently  explained.  This  green 
evidently  is  the  colour  which  is  most  usefully  employed 
in  making  colour  mixtures  in  order  to  obtain  the  three 


66  Colour  Vision. 

sensation  curves  of  the  Young  theory,  since  white  can 
be  added  to  the  colour  matched.  To  avoid  verbiage, 
we  shall  call  the  points  where  the  red-  or  green- 
blind  see  a  grey  in  the  spectrum  their  neutral 
points,  and  the  grey  they  see  at  those  points  their 
neutral  colours.  The  three  curves  we  shall  call  the 
red,  green,  or  violet  curves,  and  the  slits,  when 
placed  in  the  red,  green,  or  violet  of  the  spectrum, 
as  the  red,  green,  and  violet  slits. 

We  have  already  mentioned  the  case  of  those  who 
possess  monochromatic  vision,  and  shown  in  what 
respect  they  will  differ  in  their  description  of  the 
spectrum  from  those  more  common  cases  of  defective 
vision.  If  the  visual  sensation  they  possess  be  the 
violet,  they  will  see  no  light  at  the  extreme  red  of 
the  spectrum,  and  very  little  in  the  orange.  They 
must  match  every  colour  with  some  shade  of  grey, 
for  they  will  only  perceive  that  sensation,  in  what  to 
ordinary  normal  eyes  is  white.  We  need  not  detail 
how  those  who  possess  monochromatic  vision  due  to 
some  other  sensation  would  describe  the  different 
colours.  The  diagram  will  tell  us.  Suffice  it  to 
say,  that  one  colour  will  only  differ  from  another 
and  from  white  in  brightness. 

It  is  a  very  remarkable  fact  how  many  people  who 


I 


Examples  of  Colour  Blindness,  67 

are  defective  in  colour  vision  pass  through  a  good 
part  of  their  lives  without  being  definitely  aware  of  it. 
It  is  very  doubtful  whether,  in  the  majority  of  cases, 
they  themselves  discover  it.  They  may  quite  possibly 
attribute  the  descriptions  of  colour  which  they  hear, 
and  which  appear  to  them  absolutely  false  or  meaning- 
less, as  due  to  mental  or  moral  defects  in  their  friends. 
I  have  had  two  cases  of  this  recently.  One  was  a 
gentleman  of  seventy-four,  who  had  no  conception 
that  he  had  anything  but  normal  colour  vision  ; 
his  daughters,  however,  had  a  suspicion  that  some- 
thing was  not  quite  right  in  it,  and  after  a  good 
deal  of  persuasion  brought  him  to  me  to  examine. 
The  first  mistake  that  he  made  was  to  state  that 
he  was  sitting  on  a  black  velvet  chair,  whereas 
the  seat  was  a  deep  crimson  plush.  He  laughed 
at  his  daughter's  description  of  the  mistake  he 
made,  and  declared  he  was  only  colour  ignorant,  and 
that  she  was  the  one  who  was  colour  blind.  The 
examination  showed  that  colour  ignorant  he  was,  but 
that  the  ignorance  was  due  to  complete  red-blindness. 
For  the  seventy-four  years  he  had  lived  he  was  un- 
aware of  his  deficiency,  suspecting  it  in  others,  and 
it  was  only  an  accidental  circumstance  which  made  him 
acquainted  with   the  true  state  of  his   colour   percep- 

F    2 


68  Colour   Vision, 

tion.      Another  elderly   gentleman,  in  a  high  position 
in   life,    was   also  accidentally  tested,  and   he   proved 
to   be   completely    green-blind.       He,    too,    was    quite 
unaware  of  his  defect,  and  protested  that,   yachtsman 
as  he    was,  he  would  never   mistake   a   ship's   lights ; 
but    a    very   brief   test   showed   his  friends  who  were 
with   him    that   his   declaration   had     to    be    received 
with  a  certain   amount  of  reservation.     Others  there 
are   who    certainly   do    know    that    some    peculiarity 
exists  in  their  sense  of  colour,  and,  foolish  as  it  may 
appear  to  be — though,  after  all,  it  is  quite  consistent 
with  a  sensitive  nature — they  have  tried  to  hide  their 
defect  from  their  fellow-creatures.     Such  examples,  no 
doubt,    some    of  my    audience    have   met    with,    and 
experience    tells    me    that    they    have    just    as   much 
reluctance  to  pass  an  hour  in   my  darkened  room  as 
they  would   have   to    occupy  a   police    cell.     In  those 
few    cases    that   have    come   voluntarily    to     me    for 
examination,  the  peculiarity  in  colour  sense  was  first 
brought   to    notice   by  the  patient — if  patient  I  may 
call  him — failing  to  distinguish  between  cherries  and 
the  cherry  leaves,  or  strawberries  and  the  strawberry 
leaves.     Such  mistakes  committed  publicly  are  usually 
the  source  of  unbounded  merriment  and    curiosity  to 
schoolboys  when  made  by  their  schoolfellows,    and   I 


Maxwell's  Curves  for  Red  Blindness,  69 

am  bound  to  say  that  even  persons  of  graver  years  are 
not  unapt  to  be  amused  at  what  they  consider  to  be 
a  shortcoming  in  their  fellow-creatures.  To  the  student 
of  colour  vision  the  discovery  of  curious  cases  of  colour 
deficiency  is  looked  upon  in  a  very  different  light 
— a  good  case  of  colour  blindness,  or  still  better  one  of 
monochromatic  vision,  is  eagerly  sought  after,  with  the 
hope  of  submitting  it  to  a  rigid  examination.     When 

Fig.  18. 


we  look  at  the  diagram  (Fig.  16)  we  shall  find  why  it 
is  that  the  colour  blind  describe  the  spectrum  as  they 
do.  Literally  for  those  whose  vision  is  di-chromic,  it  is 
made  up  of  two  sensations  alone,  and  the  colours  to 
which  these  sensations  give  rise  are  mixed  throughout 
a  large  part  of  the  spectrum,  the  pure  unmixed  sensa- 
tions being  at  each  end  of  the  spectrum  as  they  are  in 
normal  colour  vision.  The  annexed  diagram  (Fig.  18) 
gives  the  curves  for  a  red-blind  person  as  made  by 
observations   under   Clerk    Maxwell's   directions.     The 


70  Colour   Vision, 

standard  colours  here  have  been  badly  selected,  for  one 
of  them  stimulates  the  two  sensations  possessed. 

An  easy  and  instructive  experiment  can  be  made  to 
give  an  idea  of  the  kind  of  colour  that  these  colour 
blind  imagine  as  white,  whether  they  be  red-,  green-, 
or  violet-blind.  (For  those  who  have  only  mono- 
chromatic vision,  as  before  stated,  white  is  coloured 
with  the  one  colour  they  possess.)  Three  slits  are  now 
in  the  spectrum,  one  near  the  extreme  end  of  the  red, 
another  well  in  the  violet,  and  the  third  in  that  part  of 
the  spectrum  in  which  the  green-blind  see  their  neutral 
colour  (see  page] 66).  With  the  three  colours  issuing 
from  these  apertures  a  match  is  made  with  the  white 
patch,  and  in  this  case  the  match  is  made  as  seen 
from  a  distant  point,  so  that  the  resulting  deductions 
may  be  true  to  the  audience.  If  a  colour-blind 
person  be  in  this  theatre,  he  will  agree  with  me 
that  the  match  is  as  correct  to  him  as  it  is  to 
myself  and  the  rest  of  you.  So  far  we  could  not 
distinguish  his  colour  perception  from  the  normal, 
but  if  he  be  red-blind,  and  the  red  slit  be  covered,  he 
will  still  say  that  the  match  holds  good,  for,  as  a 
matter  of  fact,  the  red  with  which  we  helped  to 
build  up  the  white  is  non-existent  to  him.  The 
white   that   he   now   sees  is   to   us    the    greenish-blue 


The  Neutral  Colours  of  the  Colour  Blind.       7 1 

patch  which  the  mixed  violet  and  green  make. 
If  he  be  a  green-blind  person  he  will  tell  us  the 
colour  is  a  very  pale  blue,  but  when  the  green  slit 
is  covered  up  and  the  red  uncovered,  the  match  will 
once  more  be  correct,  though  the  purple,  formed  by 
the  mixture  of  red  and  blue,  will  appear  to  him  to 
be  a  little  darker  than  the  white.  This  is  what  one 
would  expect,  for  you  must  recollect  this  green  in 
the  spectrum  he  would  call  white  or  grey.  If  then, 
from  what  to  him  is  also  white,  though  formed  by 
the  rays  coming  through  the  three  slits,  we  take 
away  a  certain  amount  of  degraded  white  (green  to 
us),  he  must  still  see  white,  but  darker.  We  have, 
however,  met  with  what  is  an  apparent  paradox. 
The  green,  coming  through  the  now  covered  slit,  he 
calls  white,  as  he  also  does  the  purple.  To  impress 
this  point  more  strongly  upon  you,  I  will  place  in 
front  of  the  green  slit  a  small  prism  which  has  an 
angle  of  about  one  and  a-half  degrees.  This  is 
just  sufficient  to  throw  the  green  colour  on  the 
neighbouring  white  surface.  Here  we  have  both  the 
.colours  which  the  green-blind  calls  white  side  by 
side.  If  the  brightness  of  each  be  the  same,  he 
would  see  no  difference  in  them.  Is  it  possible  that 
on    any    theory   this    can    be   correct  ?       To    explain 


I 


72  Colour   Vision. 

this  apparent  paradox,  and  without  reference  to  the 
mathematical  proof  that  white  subtracted  from  white 
leaves  white,  we  have  only  to  look  at  our  diagram 
(Fig.  16),  and  it  is  immediately  apparent  how  it 
arises.  The  red  and  the  blue  curves  cut  at  this 
point ;  and  if  we  take  away  the  green  sensation 
entirely,  the  residue  will  be  a  mixture  of  the  red  and 
blue,  which  is  the  identical  purple  colour  forming 
the  patch. 

If  we  are  wishful  to  ascertain  the  colour  that  the 
violet-blind  calls  white,  we  have  only  to  cover  up  the 
violet  slit  and  a  yellow  is  left  behind  as  the  result. 
I  would  have  you  remark  that  these  colours  which 
are  seen  as  white  would  only  be  of  the  hues  shown 
you,  supposing  the  colour  sensations  were  identical 
with  those  in  normal  vision.  Whether  this  is  the 
case  we  cannot  absolutely  say,  and  the  only  way  in 
which  this  can  be  authoritatively  settled  is  by 
examining  some  person  who  has  normal  colour  vision 
in  one  eye  and  defective  colour  sense,  not  due  to 
disease,  in  the  other.  One  such  person  has  been  exa- 
mined abroad,  but  in  what  way  I  am  unable  to  say. 
It  is  recorded  that  he  sees  the  red  end  of  the  spectrum 
as  yellow  with  the  eye  that  is  defective.  Another 
person  I  have  heard  of  in  England,  but  so  far  have  not 


Violet  Blindness.  73 

had  the  good  fortune  to  get  hold  of  him  for  examina- 
tion. When  I  can  lay  my  hands  on  him,  he  will  be 
able  to  help  to  confirm  or  disprove  what  should  be  a 
general  rather  than  a  particular  case. 

So  far  I  have  only  met  with  what  appears  to  be 
one  genuine  case  of  violet  blindness.  It  is  very 
remarkable,  on  account  of  the  eccentricity  of  the 
colour  nomenclature.  The  only  two  colours  which  the 
subject  saw  were  red  and  hlack.  He  named  all  greens 
and  blues  as  black,  the  distinction  between  the  two 
being  that  the  former  was  "  bright  black "  and  the 
latter  "  dark  black."  Yellow  he  called  white,  and 
a  glance  at  Fig.  16  will  show  that  at  this  place  in 
the  spectrum  the  neutral  point  of  a  violet-blind 
should  occur.  By  shifting  the  slit  gradually  into 
the  green,  he  called  it  grey,  instead  of  "  bright  black," 
though  it  did  not  match  the  white  patch  when 
darkened.  He  called  a  green  light  a  "  bright  black  " 
light.  We  shall  have  to  refer  to  this  case  when  we 
are  describing  other  investigations. 


(  74) 


CHAPTEE    VL 

Another  mode  of  exhibiting  colour  blindness,  and 
one  of  the  first  adopted,  is  by  making  mixtures  of 
colours  with  rapidly  rotating  colour  discs.  In  my 
own  experiments  I  have  chosen  a  red,  which  is 
scarlet,  over  which  a  wash  of  carmine  has  been 
brushed.  It  has  a  dominant  wave-length  of  6300. 
The  green  is  an  emerald-green,  and  has  a  dominant 
wave-length  of  5150.  The  blue  is  French  ultra- 
marine, with  a  dominant  wave-length  of  4700.  The 
card  discs,  of  some  4  inches  diameter,  are  coated 
with  these  colours  as  pastes,  and  by  making  an 
incision  in  them  radially  to  the  centre,  as  before 
described,  and  inter-locking  them,  the  compound 
disc  can  be  caused  to  show  sectors  of  any  angle 
that  may  be  required.  Outside  these  are  the  discs 
of  black  and  white,  the  proportions  of  which  can 
be  altered  at  will. 

The    light    thrown    on    the    rotating    sectors    being 


Matches  with  Discs.  75 

that  from  an  electric  arc  liglit,  normal  vision 
requires  118°  of  red,  146°  of  green,  and  96°  of  blue  to 
match  a  grey  made  up  of  75  parts  of  white  and 
285  parts  of  black.  For  the  last  two  numbers  a  cor- 
rection has  been  made  to  allow  for  the  small  amount 
of  white  light  reflected  from  the  black  surface.  This 
correction  has  also  been  made  in  the  subsequent 
matches  which  will  be  described.  Colour  mixtures 
such  as  these  are  conveniently  put  in  the  form  of  equa- 
tions, and  that  given  will  then  be  shown  as  follows — 

118  K  4-  146  G  -f-  96  U  =  75  W  -f  285  B. 
(Here  R,  G,  U,  W,  and  B  are  used  to  indicate  Red, 
Green,  Blue,  White,  and  Black.) 

This  match  was  exact  also  for  all  the  colour  blind, 
for  the  deficiency  in  one  grey  is  also  a  deficiency  in 
the  other.  With  a  red-blind,  however,  very  different 
matches  can  be  made,  as  the  red  pigment  is  a 
complex  colour.  There  is  in  it,  besides  red,  a  certain 
amount  of  yellow,  whilst  in  the  green  there  is, 
besides  green,  a  small  amount  of  a  red  and  a 
larger  amount  of  yellow.  The  yellow  will  not 
only  stimulate  the  green  sensation,  but  also  the  red 
where  it  is  present.  Although  in  complete  red-blind- 
ness the  red  sensation  is  totally  absent,  we  may  expect 
that  a  mixture  of  red  and  blue,  as  well  as  of  green  and 


"j^  Colour  Vision. 

blue,  will  enable  a   match    to   be   made    of  the   grey 
produced  by  the  mixture  of  white  and  black. 

This  was  the  case.     We  have  the  following  propor- 
tions—       295  E  +  65  U  =  45  W  4-  315  B. 

"When  the  green  disc  is  substituted  for  the  red,  the  red- 
blind  made  the  following  mixture — 

229  G  -I-  131  U  =  120  W  -h  240  B. 
It  is  worth  noticing  that  the  amount  of  blue  in  the 
first  mixture  is  about  half  that  required  for  the 
second.  This  tells  us  that  the  amount  of  green  sen- 
sation stimulated  in  the  first  case  is  much  less  than 
in  the  second.  As  red  can  be  substituted  for  green, 
it  should  follow  that  green,  when  rendered  darker, 
should  match  the  red.  To  try  this  a  red  disc  re- 
placed the  black  disc,  and  a  black  disc  replaced  the 
blue.     The  following  match  was  then  made — 

131  G  +  229  B  =  340  E  +  20  W. 
It  seems   impossible    to    believe    that    these  mixtures, 
so  dissimilar  in  colour,  could  ever  form  a  satisfactory 
match.     This  last  ecpation  might  have  been  derived 
from  the  two  first,  in  which  case  it  would  have  stood — 

137  G  -f  223  B  =  342  E  -f  18  AV. 
By  a  completely  green-blind  the  following  mixtures 
were  made — 


Matches  with  Discs,  "jj 

251  K  +  109  U  =  62  W  +  298  B,      . 

and 
277  G  +  83  U  =  107  W  +  253  B. 

In  this  case  363  Green  are  equivalent  to  251  parts  of 
Red  mixed  with  78  of  White  and  34  Black.  The 
difference  in  the  matches  made  by  the  two  types 
of  colour  blindness  is  very  evident.  In  the  one 
case  the  amount  of  red  required  is  much  greater 
than  the  green,  and  in  the  other  vice  versd.  Another 
instance  may  be  given  of  colour  matches  made,  by 
means  of  discs,  by  a  partially  green-blind  person, 
whose  case  will  be  more  fully  described  when  we 
treat  of  the  luminosity  of  the  spectrum  to  the  different 
classes  of  colour  vision. 

His   matches   were   as   follows  —  1st,    That   of    the 
normal  vision.     2nd, — 

160  E  +  80  G  +  120  U  =  72  W  4-  288  B. 
The  green  was  then  altered  to  200,  when  the  following 
made  a  match — 

65  E  +  200  G  +  95  U  =  72  W  +  288  B. 
Using  these  two  equations,  we  have  the  following 
curious  result — that  120  G  was  matched  by  95  E  + 
25  U.  As  the  green  disc  is  nearly  twice  as  luminous 
as  the  red  to  normal  colour  vision,  this  equation 
confirms    the     result     otherwise     obtained,    that     his 


78  Colour   Vision. 

blindness  to  colour  is  a  deficiency  in  the  green  sensa- 
tion. No  mixtures  of  blue  and  red,  or  blue  and  green, 
would  match  a  grey  formed  by  the  rotation  of  the  black 
and  white  sectors. 

I  must  now  introduce  to  your  notice  a  different 
method  of  experimenting  with  colour  vision.  If  we 
throw  the  whole  spectrum  on  the  screen,  and  ask  a 
person  with  normal  vision  to  point  out  the  brightest 
part,  he  will  indicate  the  yellow,  whilst  a  red-blind 
will  say  the  green,  and  so  on.  This  tells  us  that  the 
various  types  of  colour  blind  must  see  their  spectrum 
colours  with  luminosity  diiffering  from  that  of  the 
normal  eye.  The  dijBference  can  be  measured  by  caus- 
ing both  to  express  their  sense  of  the  brightness  of  the 
different  parts  of  the  spectrum  in  terms  of  white  light, 
or  of  one  another.  Brightness  and  luminosity  are  here 
used  synonymously.  On  the  two  small  screens  are  a  red 
and  a  green  patch  of  monochromatic  light — a  look  at 
the  green  shows  that  it  is  much  brighter  than  the  red. 
Eotating  sectors,  the  apertures  of  which  can  be  opened 
or  closed  at  pleasure  during  rotation,  are  now  placed  in 
the  path  of  the  green  ray.  The  apertures  are  made 
fairly  small,  and  the  green  is  now  evidently  dimmer 
than  the  red.  When  they  are  well  open  the  green  is 
once   more   brighter.     Evidently  at  some  time  during 


Luminosity  of  the  Spectrum.  79 

the  closing  of  tlie  apertures  there  is  one  position  in 
which  the  red  and  green  must  be  of  the  same  bright- 
ness, since  the  green  passes  through  the  stage  of  being 
too  light  to  that  of  being  too  dark.  By  gradually 
diminishing  the  range  of  the  ''  too  open "  to  "  too 
close  "  apertures  we  arrive  at  the  aperture  where  the 
two  colours  appear  equally  bright.  The  two  patches 
will  cease  to  wink  at  the  operator,  if  we  may  use 
such  an  unscientific  expression,  when  equality  in 
brightness  is  established.  This  operation  of  equalising 
luminosities  must  be  carried  out  quickly  and  with- 
out concentrated  thought,  for  if  an  observer  stops  to 
think,  a  fancied  equality  of  brightness  may  exist,  which 
other  properly  carried  out  observations  will  show  to  be 
inexact.  Now,  instead  of  using  two  colours,  we  can 
throw  on  a  white  surface  a  w^hite  patch  from  the 
reflected  beam,  and  a  patch  of  the  colour  coming 
through  the  slit  alongside  and  touching  it.  The  white 
is  evidently  the  brighter,  and  so  the  sectors  are  placed 
in  this  beam.  The  luminosity  of  (say)  a  red  ray  is  first 
measured,  and  the  white  is  found  to  require  a  certain 
sector  aperture  to  secure  a  balance  in  brightness.  We 
then  place  another  spectrum  colour  in  the  place  of  the 
first,  and  measure  off  in  degrees  the  brightness  of  this 
colour  in  terms  of  white  light,  and  we  proceed  similarly 


I 


8o  Colour  Vision, 

for  the  others.  Now  how  are  we  to  prove  that  the 
measures  for  luminosity  of  the  different  colours  are 
correct  ?  Let  us  place  three  slits  in  the  spectrum,  and 
by  altering  the  aperture  of  the  slits  make  a  mixture  of 
the  three  rays  so  as  to  form  white.  The  intensity  of 
this  white  we  can  match  with  the  white  of  the  reflected 
beam.  We  can  then  measure  the  brightness  (lumi- 
nosity) of  the  three  colours  separately,  and  if  our 
measures  are  correct  there  is  'prima  facie  reason  to 
suppose  that  they  will  together  make  up  the  brightness 
of  the  white.  Without  going  through  this  experiment  it 
may  at  once  be  stated  that  the  reasoning  is  correct,  for 
within  the  limits  of  error  of  observation  they  do  so. 
Having  established  this  proposition,  we  can  next  com- 
pare inter  se,  the  brightness  of  any  or  all  of  the  rays 
of  the  spectrum  by  a  preliminary  comparison  with  the 
reflected  beam  of  white  light.  As  in  the  colour  patch 
apparatus  all  colours  and  principal  dark  lines  of  the 
solar  spectrum  are  known  by  reference  to  a  scale, 
in  making  a  graphic  representation  of  the  results, 
we  first  of  all  plot  on  paper  a  scale  of  equal  parts, 
and  at  the  scale  number  where  a  reading  is  made, 
the  aperture  of  the  sectors  in  degrees  is  set  up.  Thus, 
suppose  with  red  light  the  scale  number  which  marked 
the  position  of  the  slit  was  59,  and  the  aperture  10°, 


Luminosity  Curves.  8i 

we  should  set  up  at  that  scale  number  on  the  paper 
a  height  of  10  on  any  empyric  scale.  If  in  the  green 
at  scale  No.  38  the  sectors  had  to  be  closed  to  7°, 
we  should  set  up  7  at  that  number  on  the  scale. 

When  observations  have  been  made  at  numerous 
places  in  the  spectrum,  the  tops  of  these  ordinates,  as 
they  are  called,  should  be  joined,  and  we  then  get  the 
observed  curve  of  luminosity  for  the  whole  spectrum. 
For  convenience'  sake  we  make  the  highest  point  100, 
and  reduce  the  other  ordinates  in  proportion.  For 
some  purposes  it  may  be  advantageous  to  give  the 
luminosity  curve  in  terms  of  a  scale  of  wave- 
lengths. For  our  purpose,  however,  it  is  in  general 
sufficient  to  use  the  scale  of  the  instrument. 

Now,  if  we  test  the  vision  of  the  various  types  of 
colour  blind  by  this  plan,  we  should  expect  to  get 
luminosities  at  different  parts  of  the  spectrum  which 
would  give  very  different  forms  to  these  curves.  We 
cannot  hope,  for  instance,  that  a  red-blind  who  sees  no 
red  in  the  extreme  end  of  the  spectrum  would  show  any 
luminosity  in  that  region,  nor  that  the  green-blind 
should  show  as  much  in  the  green  part  of  the  spectrum 
as  those  who  possess  normal  colour  vision,  since  one  of 
the  sensations  is  absent.  With  monochromatic  vision 
there    should   be   a   still   further   departure    from   the 


82 


Colour  Vision. 


normal  curve.     That  these  differences  do  exist  is  fully 
shown  in  Fig.   19.     One  of  the  most  striking  experi- 

FiG.  19. 


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ments  in  colour  vision  is  to  place  a  bright  red  patch  on 
the  screen,  and  to  ask  a  red-blind  to  make  a  match  in 


I 


Partial  Colour  Blindness.  83 

luminosity  with  the  white.  The  latter  will  have  to  be 
reduced  to  almost  darkness — a  darkness,  indeed,  that 
makes  the  match  almost  seem  incredible.  You  will 
notice  that  the  places  in  the  spectrum  where  the 
red-  and  green-blind  see  grey  are  by  no  means  places 
of  greatest  luminosity.  We  shall  find  that  these 
luminosity  curves  are  suggestive  when  making  another 
investigation  into  the  form  of  the  spectrum  curves 
of  the  colour  sensations. 

Besides  cases  of  complete  blindness  due  to  the 
absence  of  one  or  two  sensations  on  the  Young  theory, 
we  have  other  cases,  as  was  said  when  remarking  on  the 
percentage  of  people  who  are  colour  deficient,  in  which 
one  or  even  two  sensations  are  only  more  or  less 
deadened.  It  has  often  been  said  that  with  the  theory 
provisionally  adopted,  such  cases  are  difficult  to  class  as 
red  or  green  deficient.  As  far  as  my  own  observations 
go,  I  have  never  found  this  difficulty.  The  luminosity 
curves  of  such  observers,  combined  with  other  indica- 
tions, give  a  ready  means  of  classing  them.  The  main 
difficulty  to  my  mind  is  to  state  what  is  normal 
colour  vision,  but,  as  I  have  found  that  the  very  large 
majority  of  eyes  give  the  same  luminosity  to  colours  as 
my  own,  I  have  taken  my  own  colour  perception  as 
normal.       In     numerous      experiments     which      Lord 

G  2 


84  Colour  Vision, 

Kayleigh  has  made  in  matching  orange  by  means  of 
a  mixture  of  red  and  green,  he  has  come  across  several 
who  have  apparently  normal  vision,  as  they  see  colours 
correctly  in  every  part  of  the  spectrum,  and  yet  some 
require  much  less  red  mixed  with  the  green  to  make  a 
match  with  the  orange  than  do  others.  What  is  yellow 
to  them  is  decidedly  green  to  the  majority.  This  has 
been  classed  as  another  kind  of  normal  vision  ;  but  the 
luminosity  curves  show  that  it  may  be  equally  well 
due  to  a  deficiency  in  the  green  sensation,  and 
which  would  require  more  green  to  make  the 
necessary  match.  The  limits  of  the  visible  spectrum 
to  these  persons,  as  far  as  my  examination  of  their 
cases    goes,  are  the  same   as  my  own. 

Again,  there  are  others  in  which  the  spectrum 
seems  decidedly  somewhat  shortened  at  the  red  end 
compared  with  my  own,  and  the  luminosity  curves 
point  to  them  as  being  strictly  colour  deficient  in  the 
red  and  nothing  else.  As  they  see  all  colours,  they 
have  been  classed  as  another  form  of  normal 
vision.  The  deficiency  in  both  these  cases  is  so 
small  that  white  is  their  neutral  colour,  but  there 
is  evidence  that  the  hues  are  slightly  changed.  I 
do  not  wish  any  one  to  accept  my  deductions  as  being 
more  correct  than  those  who  hold  differently,  but  the 


Partial  Colour  Blindness.  85 

results  of  examination  by  the  luminosity  methods 
appear  to  me  difficult  to  reconcile  with  any  other  view. 
There  are,  however,  a  large  number  of  cases  in  which, 
though  complete  red-  or  green-blindness  is  wanting, 
there  is  no  doubt  that  more  than  slight  colour 
deficiency  exists.  For  instance,  in  Fig.  23  we  have 
the  curve  of  luminosity  of  the  spectrum  as  measured 
by  a  very  acute  scientific  observer,  and  it  is  compared 
with  that  of  normal  colour  vision.  He  certainly  is 
not  completely  blind  to  any  sensation.  An  inspec- 
ti(m  and  comparison  of  the  two  curves  will  show 
that  he  is  defective  in  the  green  sensation,  although  it 
is  present  to  a  large  extent.  The  deficiency  is  obvious 
enough.  An  endeavour  to '  find  his  neutral  point  was 
most  interesting.  At  39  in  the  scale  he  saw  a  little 
colour,  but  at  39*5  all  colour  had  vanished,  and 
between  the  coloured  patch  and  the  white  he  saw  no 
difference.  This  similarity  he  saw  till  47  *  3  in  the 
scale,  w^hen  he  began  to  see  a  faint  trace  of  colour. 
There  is  a  large  piece  of  the  spectrum,  then,  which 
to  him  is  grey.  It  must  be  recollected  that  all  three 
sensations  were  excited  in  this  region,  but  some  more 
than  others.  Now,  experiment  has  shown  that,  with 
normal    vision,  two   per  cent,    of   any   colour  may  be 

mixed  with  a  pure  colour  without  its  being  perceived. 


86  Colour   Vision. 

It  is  not  surprising,  therefore,  that  although  the  red, 
or  the  green,  or  the  blue  may  be  present  in  an  in- 

FiG.  23. 


tensity  above  that  required   to   form   white,   yet   the 
resulting  sensation  should  pass  for  white.     It  may  be 


Partial  Colour  Blindness,  87 

remarked  that  red  and  white  when  mixed  he  never 
mistook  for  yellow,  and  he  always  recognised  yellows 
and  red ;  yellowish  green,  however,  he  called  pale 
yellow. 

Another  example  of  partial  red-blindness  is  also 
instructive.  Fig.  23  also  shows  it  graphically.  There 
is  no  doubt  as  to  the  nature  of  the  defect.  The 
spectrum  is  slightly  shortened,  and  the  luminosity 
of  this  part  of  the  spectrum  is  less  than  that  of 
normal  vision.  There  was  no  difficulty  in  distin- 
guishing every  colour,  though  the  positions  of  the 
colours  from  yellow  to  green  seemed  to  be  shifted  ;  but 
no  neutral  point  could  be  traced.  Apparently,  both 
this  case  and  the  former  are  about  equally  colour 
defective  ;  but  in  this  last  the  same  reasons  do  not 
apply  for  the  existence  of  a  neutral  point.  (For 
measures   see  page  214.) 


I 


(  88  ) 


CHAPTER     VIL 

We  are  now  in  a  position  to  carry  the  investigations 
as  to  luminosity  a  little  further.  When  we  look  at 
small  patches  of  light,  we  view  the  colour  through  the 
yellow  spot  in  the  eye.  If,  when  we  have  matched 
the  luminosity  in  the  ordinary  manner,  we  turn  our 
eyes  some  10°  away  from  the  patches,  we  shall  find 
that  except  at  one  place  in  the  green  the  equality  in 
brightness  no  longer  exists.  By  a  little  practice  we  can 
make  matches  of  luminosity  when  the  eyes  are  thus 
diverted.  This  will  give  us  a  different  curve  of 
luminosity,  as  the  yellow  spot  absorption  is  absent, 
and  the  difference  in  the  heights  of  the  ordinates 
between  the  two  curves  will  give  us  that  absorption. 
Fig.  20  shows  this  very  well ;  and  it  will  be  noticed 
that  the  eye  is  appreciably  not  so  sensitive  to  the  red 
and  yellow  at  10°  from  the  axis  as  it  is  on  its  central 
area.  If  we  measure  the  areas  of  these  two  curves  w^e 
get  the  relative  values  of  the  light   energy  which   is 


Sensitiveness  of  the  Fovea  to  Colour. 


89 


active  on  the  two  parts  of  the  eye,  and  these  we  found 
to  ])e  as  167  to  156.     The  heights  at  which  to  put  the 

Fig    20. 


90  Colour  Vision. 

maxima  of  the  two  curves  were  found  from  various 
considerations,  and  the  correctness  of  the  deductions 
was  verified  by  directly  comparing  the  intensities  of  two 
patches  of  white  light  some  10°  apart,  which,  when 
looked  at  direct,  were  of  equal  intensities.  When  one 
was  compared  with  the  other,  the  eye  receiving  one 
image  centrally  and  the  other  outside  the  yellow 
spot,  the  difference  in  values  was  closely  proportional 
to  those  of  the  above  areas.  The  part  of  this  last 
curve  showing  a  deficiency  in  red  sensation  is  very 
similar  to  that  obtained  from  a  person  who  is  partially 
colour  blind.  The  absorption  by  the  yellow  spot 
derived  from  these  measures  is  graphically  shown  in 
the  next  figure  (Fig.  21). 

The  question  of  the  visual  sensation  at  the  ''  fovea 
centralis  "  (if  it  be  admitted  that  this  is  coincident  with 
the  visual  axis  of  the  eye,  as  is  usually  accepted)  may 
be  very  easily  studied.  When  the  luminosity  of  the 
spectrum  is  examined  at  five  or  six  feet  distance,  by 
throwing  the  two  patches  on  the  whitened  face  of  a 
small  square  of  half-inch  side,  we  get  a  result  differing 
from  both  of  the  above.  The  fovea  appears  to  be 
slightly  more  sensitive  to  red  than  the  macula  lutea, 
and  is  generally  less  sensitive  to  the  green  rays  (see 
Fig.  20).     If  a  star,  or  a  distant  light,  be  observed  with 


Absorption  by  the  Yellow  Spot.  91 

the  part  of  the  retina,  on  which  the  axis  of  the  eye  falls, 
as  is  the  case  in  ordinary  vision,  and  then  be  observed 


Fig.  21. 


92  Colour   Vision. 

with  the  eye  slightly  directed  away,  the  difference 
in  the  colours  of  the  light  is  unmistakable.  (The 
tables  giving  the  measured  value  of  these  curves 
will  be  found   in    the  appendix,  page  211.) 

Can  we  in  any  way  find  from  these  methods  the 
colour  sensation  curves  ?  I  think  wt  can.  Suppose  we 
have  a  second  instrument  exactly  like  the  first  placed 
side  by  side  with  it,  we  can  then  throw  two  patches  of 
colour  on  the  two  adjacent  white  surfaces,  and  we  can 
mix  with  either,  or  both  of  them,  as  much  white  light 
as  w^e  choose.  From  the  second  instrument  let  us 
throw  all  the  spectrum  colours  in  succession  on  to 
the  one  surface,  and  on  to  the  other  the  three 
primary  colours  mixed  in  such  proportions  as  to  match 
them  accurately.  This  plan  is,  I  venture  to  think, 
a  better  way  of  obtaining  the  value  of  colours  in 
terms  of  standard  colours  than  that  adopted  by 
Maxwell.  This  method  gives  the  values  directly, 
and  not  by  calculation  from  matches  with  white. 
Let  us  place  one  slit  near  each  of  the  extreme  ends  of 
the  spectrum  ;  that  in  the  red  near  the  red  lithium 
line,  and  another  a  little  beyond  Gr  in  the  violet 
of  the  spectrum,  whilst  the  third  slit  should  be  in 
the  exact  position  in  the  green,  where  the  green- 
hlind  sees  grey.     Now  it  might  be  a  matter  of  dispute 


The  Green  Sensation.  93 

as  to  whether  one  was  entitled  to  make  this  last  one 
of  the  positions  for  the  slits,  for  we  use  it  entirely 
on  the  assumption  that  two  of  the  colour  sensations 
which  we  suppose  we  possess  are  identical  with  those  of 
the  green-blind.  This  might  be,  or  might  not  be,  the 
case ;  but  I  think  it  can  be  shown  very  easily  that  the 
assumption  we  are  making  is  more  than  probably  exact. 
Having  the  slits  in  these  positions,  we  may  endeavour 
to  match  the  spectrum  orange.  We  mix  the  red  and  the 
green  lights  together,  and  find  that  the  best  mixture 
is  always  paler  than  the  orange,  but  by  adding  a  small 
quantity  of  white  to  the  orange  we  at  once  form  a 
match.  In  the  same  way  if  we  have  a  greenish -blue 
to  match,  we  shall  find  that  we  can  only  make  the 
match  when  we  add  a  little  white  to  the  simple 
colour.  Now  let  us  shift  the  position  of  the  slit  in 
the  green  just  a  little — a  very  little — towards  the 
blue,  and  again  try  to  match  orange.  Do  what  we 
will  we  cannot  find  apertures  to  the  slits  which  will 
give  us  the  colour,  though  it  be  diluted  with  white. 
It  will  be  too  blue  or  too  red,  but  never  exactly  orange. 
This  tells  us  that  there  is  too  much  blue  in  the  green 
we  are  using.  Next,  shift  the  slit  a  little  towards  the 
red  below  our  fixed  position,  and  endeavour  to  match 
the  blue.     We  shall  find  that  this,  too,  becomes  imprac- 


94  Colour   Vision, 

ticable.  The  blue  is  either  too  green  or  too  violet, 
telling  us  that  our  mixture  contains  too  much  green. 
As  the  neutral  point  of  the  colour  blind  is  the  only 
position  for  the  green  slit  which  enables  us  to  make 
a  good  match  to  both  the  orange  and  the  blue,  it 
follows  that  this  must  be  the  point  where  these  two 
colour  sensations  are  so  arranged  as  to  be  in  the  pro- 
portions required  to  form  white  when  green  is  added  ; 
that  is,  that  there  is  neither  an  excess  of  red  nor  an 
excess  of  violet.  To  come  back  to  our  measures  of 
colour.  We  can  make  up  every  spectrum  colour  with 
these  three  colours,  and  finally  divide  the  luminosity 
curve  into  the  colour  luminosity.  In  all  these  matches 
the  violet  luminosity  is  very  small  indeed  compared 
with  the  red  or  green.  A  match  with  white  is  now 
made  by  a  mixture  of  all  these  colours,  and  you  will 
see,  from  the  images  of  the  slits  on  the  screen,  that  the 
luminosity  of  the  violet  is  almost  a  negligible  quantity 
compared  with  the  others.  We  may,  therefore,  as  a 
first  approximation,  divide  up  the  luminosity  curve  into 
two  parts,  one  being  the  luminosity  of  the  green  in  the 
different  colours  and  the  other  of  the  red.  The  green, 
however,  is  made  up  of  red,  of  violet,  and  of  an  excess 
of  green  sensation,  which  in  this  case  comes  practically 
to  a  mixture  of  white  with  the  green  sensation.     How 


Colour  Stusations  in   Terms  of  Luminosity .     95 


can  we  tell  how  much  is  green  and  how  much  is  white  ? 
Suppose  I,  as  a  normal-eyed  person,  compare  the  lumi- 
nosity of  the  colour  coming  through  the  red  slit  with 
that  coming  through  the  green  slit,  and  then  get  the 
green-blind  to  do  the  same,  it  is  evident  that  any  excess 
in  the  luminosity  as  measured  by  myself  over  that 
measured  by  the  green-blind  must  be  due  to  the  green 
sensation,  and  we  can  also  see  how 
much  red  and  violet  make  up  his 
white.  We  shall  not  be  far  wrong, 
then,  in  apportioning  the  consti- 
tuents of  the  white  thus  found 
between  the  green  and  the  red ; 
the  violet  being,  for 


Fio.  22. 


the  time  bein 


o' 


neg- 


ligible.      We    must         g 

subtract  the  red  sensation  from  the  green  colour  curve  and 
add  it  to  the  red  colour  curve  :  the  two  curves  will  then 
be  very  closely  the  curves  of  the  red  and  green  sensa- 
tions. By  causing  the  green-blind  to  make  mixtures  of 
red  and  violet  for  all  the  colours  of  their  spectrum,  we 
can  arrive  at  what  must  be  finally  taken  away  or  given 
to  these  curves,  though  such  addition  or  subtraction  of 
fHolet  will  be  small  when  the  luminosities  are  considered 
The  accompanying  figure  (Fig.  22)  gives  an  idea  of  the 


96  Colour   Vision, 

shape  and  general  features  of  these  curves.  It  may  he 
remarked  that  we  can  check  the  general  accuracy  of 
the  measures  of  the  colour  mixtures  by  calculating  or 
measuring  the  areas  of  the  two  colour  curves,  the  red 
and  the  green.  If  accurate,  they  should  bear  the  same 
ratio  that  the  luminosities  of  the  two  colours  bear  to 
each  other  (when  mixed  with  a  little  violet^  which  is 
practically  negligible)  to  form  white  light.  So  far,  then, 
we  can  utilize  the  luminosity  methods  to  calculate  and 
to  trace  the  sensation  curves  for  the  normal  eye.  It 
will  not  escape  your  notice  that  the  maximum  heights 
of  these  two  component  curves  are  nowhere  near  the 
parts  of  the  spectrum  where  the  colour  is  the  purest. 
Another  check  to  these  curves  may  also  be  made  by 
taking  the  difference  in  the  ordinates  of  the  luminosity 
curves  of  the  colour  blind  and  the  normal  eyed.  Too 
much  stress  must  not,  however,  for  the  moment,  be 
laid  on  this,  as  this  method  depends  on  the  absolute 
correctness  of  the  scale  of  the  ordinates  in  the  curves. 
It  must  be  recollected  that  to  the  former  white  light 
is  deprived  of  at  least  one  constituent  sensation  which 
is  perceived  by  normal  eyes.  This,  in  all  probability, 
renders  the  white  less  luminous  to  them  than  those 
possessing  normal  vision,  so  that  the  comparisons  of 
luminosities  are  referred  to  different  standards. 


Colour  Sensation  in   Terms  of  Luminosity.     97 

It  may  seem  a  very  simple  matter  to  ascertain  the 
correct  scale,  but  it  is  not,  except  by  the  extinction 
method,  which  will  be  described  later.  At  one  time 
General  Festing  and  myself  tried  to  obtain  a  comparison 
by  finding  the  limiting  illumination  at  which  a  book 
could  be  read.  We  got  results,  but  for  the  purpose 
in  question  the  values  are  not  conclusive.  What  we 
really  were  measuring  was  the  acuteness  of  vision  in 
different  coloured  lights.  As  a  good  deal  depends  upon 
the  optical  perfection  of  the  eyes  under  examination, 
besides  the  illumination,  we  must  be  on  our  guard,  even 

if  there  were  nothing  else  against  the  method,  against 
taking  any  such  measures  as  being  conclusive. 


H 


(  98  ) 


CHAPTEE    YIII. 

Before   quitting   the   measurement   of  luminosity,    it 
may  be  as  well  to  see  whether  the  curves  described  are 
the  same  whatever  the  brilliancy  of  the  spectrum  may 
be.     We  can  easily  experiment  with  a  very  reduced 
brightness.     Upon  the  screen  we   have  an   ordinarily 
bright  spectrum.     As  the  slit,  through  which  the  white 
light  forming  the  spectrum  comes,  is  narrowed,  there  is 
an   evident   change  in  the  relative   brightness  of  the 
different  parts,  though  the  energy  of  every  ray  must 
be   proportionally  reduced.      The    red    is    much    more 
enfeebled  than  the  green,  and  in  brightness  the  green 
part  of  the  spectrum  looks   much  more  intense  than 
the   yellow,    which   is    ordinarily   the    brightest    part. 
This    we    have    assured     ourselves    of    not    only    by 
casual    observation,    but   also  by  direct   Ineasurement* 
Perhaps  I  can  make  this  even  more  decisive  to  you. 
Two  slits  are  now  in  the  ordinarily  bright  spectrum, 
one   in   the    red,  and   the  other   in  a  green  which   is 


Red-bli7id  Spectrtim  Ltunmosity .  99 

near  the  E  line  of  the  solar  spectrum.  Instead  of 
using  one  lens  to  form  a  single  colour  patch  of  mixed 
light,  two  parts  of  a  lens,  appropriately  cut  and  of 
the  same  focal  length  as  the  large  combining  lens, 
are  placed  in  front  of  the  slits,  one  bit  of  lens  before 
each.  This  artifice  enables  us  to  throw  the  patch  of 
red  on  one  white  surface,  and  the  patch  of  green  light 
on  another  adjacent  to  it.  By  opening  or  closing  one 
or  other  of  the  slits  the  brightness  of  the  two  patches 
of  light  are  so  arranged  that  there  is  no  manner 
of  doubt  but  that  the  red  is  the  brighter  of  the 
two.  The  absolute  energies  of  the  rays  forming 
each  of  the  patches  are  proportionally  reduced  by 
closing  the  slit  of  the  collimator,  as  before.  At  one 
stage  both  patches  appear  of  about  the  same  intensity. 
This  might  be  taken  for  an  error  in  judgment,  but 
to  make  the  change  that  takes  place  perfectly  plain 
to  you,  the  rotating  sectors  are  introduced  in  front  of 
the  two  slits,  and  the  rays  now  pass  through  them. 
The  apertures  of  the  sectors  are  gradually  closed,  and 
we  now  come  to  such  a  reduction  that  the  red  is 
absolutely  invisible ;  but  the  green  still  shines  out. 
It  is  losing  its  colour  somewhat,  and  appears  of  a 
bluish  tint.  The  i-eason,  Qf  .thi^^^cbang^  of » hue  in  the 
latter  we  shall, shortly  ,se,e.  ,  The  sectors^  h^^  withdrawn 


lOO  Colour   Vision. 

and  the  red  re-appears,  and  is  as  bright  as  the  green. 
The  slit  of  the  collimator  is  next  opened,  and  there 
is  no  doubt  that  the  red  is  much  brighter  than  the 
green,  as  it  was  purposely  made  at  the  beginning  \ 
of  the  experiment.  The  same  class  of  experiment 
might  have  been  repeated  with  the  green  and  violet 
or  the  red  and  violet,  and  the  same  kind  of  results 
would  have  been  obtained.  The  violet  would  have 
been  the  last  to  disappear  when  the  green  was  so 
reduced  in  luminosity  that  it  appeared  in  the  ordinary 
brilliant  spectrum  to  be  equal  to  the  violet  ray 
selected.  When  the  green  was  of  the  luminosity 
given  by  a  slit  equal  in  width  to  that  of  the  violet,  the 
violet  would  have  disappeared  first,  owing  to  its  feeble 
brightness  to  begin  with.  Now,  if  we  measure  a  feebly 
illuminated  spectrum  we  must  adopt  some  special 
means  to  exclude  all  light,  except  that  of  the 
comparison  light  and  the  ray  to  be  measured.  This 
we  can  do  by  the  box  which  is  shown  in  the  next 
diagram  (Fig.  24). 

At  one  end  of  a  box,  shown  in  plan,  is  an  eye- 
piece, E.  The  other  end  has  at  its  centre  a  white 
square  of  paper  of  ij-inch  scale.  The  mono- 
chromatic \^^W.nCbl 'coijiiii'g  fKoiii  ^t^Q  Spectrum  through 


vV  wvv  fi^^  ^^^4i^>^^  ^^  r  reference.  „be^m  ^  *}),  pi  white  light, 


Luminosity  of  a  Feeble  Spectrum.  loi 

are  reflected   from   glass   mirrors   Mj,  Mg  to  apertures 
in   opposite  sides  of  the  box,  and  from  close  to  these 


I 

llppertures   by  the  right-angled  prisms  Pj  Pg,  so  as  to 
fall  on  and  cover  S.     Eods  Ki,  Rg  are  inserted  in  the 


I02  Colour   Vision. 

box  in  the  paths  of  the  beams,  so  that  the  opposite 
halos  of  S  are  illuminated.  Diaphragms  inside  the 
box  cut  off  any  stray  light,  and  rotating  sectors 
placed  at  A  and  B  regulate  the  intensity  of  the 
beams  as  required.  The  sector  A  is  rotated  with  a 
previously  determined-on  aperture ;  the  white  light 
coming  through  B  is  altered  till  the  luminosity  of  the 
two  on  the  screen,  as  seen  through  E,  are  the  same. 
Every  part  of  the  spectrum  can  be  measured  in  this 
way ;  the  result  is  shown  in  the  diagram.  Fig.  25 
(the  measures  will  be  found  at  page  215  in  the 
appendix).  In  this  case  the  orange  light  at  D  where 
it  fell  on  the  screen  was  equal  to  y^^  of  an  amyl- 
acetate  light,  which,  in  its  turn,  is  closely  *  8  of  a 
standard  candle.  In  the  same  figure  the  luminosity 
curve  of  the  ordinary  bright  spectrum  is  given  for 
reference,  and  it  can  be  seen  how  the  point  of 
maximum  luminosity  is  shifted  into  the  green,  lying 
almost  over  the  E  line  of  the  solar  spectrum.  The 
maximum,  of  course,  has  been  made  100  as  before,  for 
had  it  been  drawn  to  the  same  scale  as  the  other, 
the  form  of  the  curve  would  not  have  been  demon- 
strated. There  is  a  remarkable  resemblance  between 
it  and  the  curve  of  luminosity  of  the  monochromatic 
vision,    and    such    a     resemblance  ^  can     scarcely    be 


I04  Colour   Vision, 

and  is  almost  the  same  as  that  observed  when  the 
spectrum  is  reduced  to  such  an  extent  that  it  is 
colourless  throughout,  a  condition  that  it  can  assume, 
as  we  shall  see  very  shortly.  When  the  spectrum  is 
rather  more  luminous,  it  gives  a  curve  of  luminosity 
which  is  similar  to  that  of  the  ordinary  spectrum 
when  measured  by  a  red-blind  person.  Here,  then, 
we  have  an  indication  that  a  person  with  normal 
vision  passes  through  a  stage  of  red-blindness,  as  the 
intensity  is  diminished  before  he  arrives  at  absolutely 
monochromatic  vision. 

This  investigation  is  of  practical  as  well  as 
'theoretical  interest,  as  General  Festing  and  myself 
'quickly  discovered  when  we  first  made  it.  The 
curious  colour  of  a  moonlight  landscape  is  entirely 
accounted  for  by  it.  White  light  becomes  greenish- 
blue  as  it  diminishes  in  intensity,  and  the  reds  and 
yellows,  being  reduced  or  absent,  are  not  reflected  by 
surrounding  objects.  Hence,  moonlight  is  cold,  whilst 
the  sunlight  is  warm  owing  to  their  presence. 

When  measuring  these  low  luminosities,  the  various 
colours  will  in  a  great  measure  disappear.  Part  of 
the  spectrum  will  be  of  that  peculiar  grey  which 
was  shown  you  in  the  experiment  with  the  in- 
candescent   light    (p.   34).     By  further    experiment    it 


Extinction  of  Colour,  105 

is  possible  to  arrive  at  an  approximate  determination 
of  the  point  where  all  colour  vanishes  from  the 
different  parts  of  the  spectrum.  We  use  the  same 
apparatus  (Fig.  24)  as  before,  the  only  difference  being 
that  each  of  the  sectors  is  movable  during  rotation. 
The  apertures  of  those  through  which  the  colour 
passes  are  reduced  till  all  colour  on  the  screen  just 
disappears,  the  point  being  arrived  at  by  a  com- 
parison with  the  white,  which  is  itself  also  reduced. 
The  apertures  of  the  first  sector  alone  need  be  noted, 
and  from  these  readings  the  diagram  (Fig.  26)  is. 
made  (for  measures,  see  page  216). 

This  extinction  of  colour  is  one  which  often  occurs, 
but  is  seldom  noticed.  The  figure  tells  us  that  the 
orange  is  about  the  last  colour  of  the  spectrum 
left,  some  of  the  others  still  appearing  as  greys.  The 
next  to  retain  its  colour  is  the  green,  and  the  most 
rapid  to  lose  them  are  the  red  and  violet.  It  must 
not  be  supposed  that  the  colours  remain  of  the  same 
hue  up  to  the  time  that  they  vanish.  Pure  spectrum 
red  (red  sensation)  remains  the  same  up  to  the  last, 
but  the  scarlet  becomes  orange,  and  the  orange 
yellower,  and  the  green  bluer.  This  is  what  would 
be  predicted  from  the  Young  theory  if  the  order  of 
extinction    of    sensation    be    red,  green,  violet.      This 


o6 


Coloicr  Vision^ 

Fig.  2G. 


^7   \y/iu/i'  /  /^/^^,>, 


First  Appearance  of  Colour,  107 

we  shall  see  is  the  case.  At  nightfall  in  the 
summer  the  order  of  disappearance  of  colour  may 
often  be  seen  ;  orange  flowers  may  be  plainly  visible, 
yet  a  red  geranium  may  appear  black  as  night ;  the 
green  grass  will  be  grey  when  the  colour  of  the 
yellow  flowers  may  yet  be  just  visible.  An  early 
morning  start  in  the  autumn  before  daybreak  will 
give  an  ample  opportunity  of  satisfying  oneself  as  to 
the  order  in  which  colours  gradually  re-appear  as 
daybreak  approaches.  Ked  flowers  will  be  at  the  out- 
set black,  whilst  other  colours  will  be  visible  as  grey. 
As  more  light  comes  from  the  sky  the  pale  yellow 
and  blue  flowers  will  next  be  distinguished,  though 
the    grass   may  still    be    a    nondescript    grey.     Then^ 

as  the  light  still  increases,  every  colour  will  burst 
out,  if  not  in  their  full  brilliance,  yet  into  their  own 
undoubted  hue. 


I 


(    io8  ) 


CHAPTER     IX. 

Not  only,  however,  may  we  lose  a  sense  of  colour, 
but  we  may  also  lose  all  sense  of  light  by  reducing 
the  energy  of  the  different  rays.  We  have  seen  that 
Qolour  goes  unequally  from  the  different  parts  of 
the  spectrum.  We  may  therefore  prognosticate  that 
the  light  itself  may  disappear  more  rapidly  from  some 
parts  than  from  others.  You  will  scarcely,  however,  I 
think,  be  prepared  for  the  enormous  difference  which 
exists  in  the  stages  of  disappearance  of  the  grey  of 
the  reduced  red  and  of  that  of  the  reduced  green. 

But  how  are  we  to  measure  this  extinction  of  light 
at  the  different  parts  of  the  spectrum?  This  is  a 
problem  which  I  have  attacked  during  the  last  few 
years  by  a  variety  of  methods ;  but  as  is  the  case 
with  almost  every  scientific  problem,  when  the  mode 
of  attack  is  reduced  to  its  simplest  form,  it  yields 
the  more  readily  to  solution.  If  we  have  a  box,  like 
that   figured    in    Fig.    27,    and   combine    it   with    our 


Fm.  27. 


B 


Extinction  of  Light.  lOo 

colour  patch  apparatus,  the  problem  is  solved.  B  B 
is  a  closed  box  3  feet  long  and  about  1  foot  high 
and  wide,  having  two  similar  apertures  \\  inch  in 
diameter  in  the  positions  shown.  The  aperture  at 
the  side  is  covered  on  the  inside 
by  a  piece  of  glass  a,  ground  on 
both  sides,  and  a  tube  T  is  in- 
serted, in  which  diaphragms,  D,  of 
various  apertures  can  be  inserted 
at  pleasure.  The  most  convenient 
form  of  diaphragm  is  that  supplied 
with  photographic  lenses — an  iris 
diaphragm.     E    is    a    tube    fitted 

at    the    end 
^  /^  of    the    box 

through 
which  the 
screen  S  is  viewed.  S  is  black 
except  in  the  centre,  where  a 
white  disc  is  fastened  to  it.  A 
mirror,  M,  placed  as  shown,  reflects  the  light  scattered 
by  the  ground  glass  on  to  the  screen  S.  The  rotating 
sectors  are  placed  where  shown,  and  are  in  such  a 
position  that  they  can  be  readily  adjusted  by  the 
observer.      The   patch   of  any   desired   colour   of   the 


% 


no  Colour   Vision, 

spectrum  is  thrown  on  a,  and  an  appropriate  size 
of  diaphragm  used,  so  that  when  the  sectors  are  not 
less  than  5°  to  10°  open,  the  light  totally  disappears. 
We  can  now  make  observations  throughout  the  whole 
spectrum,  and  knowing  the  value  of  the  different 
apertures  of  the  diaphragm  and  the  angular  opening 
of  the  rotating  sectors,  we  can  at  once  find  the 
amount  of  reduction  of  the  particular  part  of  the 
spectrum  that  is  being  required  in  order  to  just 
extinguish  all  traces  of  light  from  the  white  disc 
at  the  end  of  the  box.  From  these  measures 
we  can  readily  construct  a  curve  or  curves  which 
will  graphically  show  the  reduction  given  to  the 
different  parts  of  the  spectrum.  Fig.  28  gives  the 
curve  of  extinction  for  ordinary  normal  colour  vision. 
The  spectrum  was  of  such  a  brilliance  that  the 
intensity  of  the  square  patch  of  light  formed  on  a 
of  the  orange  light  (D)  was  exactly  that  of  an 
amyl-acetate  lamp,  placed  at  one  foot  distance  from 
the  receiving  screen.  Knowing  this,  the  actual  lumi- 
nosity of  all  the  other  rays  of  the  spectrum  can  be 
derived  from  the  curve  of  luminosity  (see  Fig.  20). 
Extinguishing  the  various  parts  of  the  spectrum 
by  this  plan,  it  is  found  that  the  red  rays  cease 
to    stimulate     the    retina     sufficiently    to     give    any 


J 


Extinction  of  Light,  1 1 1 

appearance  of  light  long  before  the  green  rays  are  ex- 
tinguished.    It  is  only  the  rays  in  the  extreme  violet 

riG.2s. 


-^m^'mar-m 


■T^M'/M 


SBaHB 


^il^^=^ 


■■■E9B 
■■■■SB 


[grii 


i)f    the    spectrum,    and    which    consequently    possess 
very    feeble    luminosity,    that    make    any    approach 


1 


112  Colour  Vision, 

towards  requiring   the   same    amount  of   reduction  as 
the  red  rays. 

There  is  the  fact  to  remember  in  making  these 
measures  in  the  extreme  red  and  the  extreme  violet, 
that  the  luminosities  of  the  colours  are  so  small  that 
the  illumination  of  the  prism  itself,  by  the  white  light 
falling  on  it,  has  to  be "  taken  into  account,  since  it 
forms  an  appreciable  portion  of  the  patch  of  feeble 
colour.  By  placing  a  proper  shade  of  blue  or  red  glass 
in  the  front  of  the  collimator  slit  this  white  light  dis- 
appears or  becomes  negligible,  and  when  the  absorption 
of  the  coloured  glass  is  known  from  measurement, 
we  can  get  a  very  accurate  measure  of  the  extinction 
of  these  parts.  Some  people  may  propound  the  idea 
that  the  rotating  sectors  may  in  such  kind  of  measure- 
ments give  a  false  result.  Now  such  a  criticism  is 
quite  fair,  and  it  is  absolutely  necessary  that  it  should 
be  answered.  Well,  to  test  the  accuracy  or  the  reverse 
of  the  assumption  that  such  measures  are  correct,  the 
following  small  piece  of  simple  apparatus  was  devised. 
A  and  B  (Fig.  29)  are  two  mirrors  placed  at  angles 
of  45°  to  the  angle  of  incidence  of  the  beam.  The 
path  the  beam  takes  can  be  readily  ascertained  from 
the  figure.  This  piece  of  apparatus  was  placed  in 
position   in   front  of  the  spectrum,  and    the   reflected 


A   Criticism  Answered,  113 

beams  used  to  form  the  patches  of  colour.  For 
convenience  only  a  small  pencil  of  light  was  allowed 
to  issue  from  the  prism,  a  diaphragm  of  some  ^-inch 
in  diameter  being  placed  in  front  of  it.  This  allows 
a   spot   of  any    desired   colour  to  Fio.  29. 

fall  on  the  screen,  the  ground 
glass  being  removed.  The  slit 
through  which  the  spectrum  colours 
pass  is  moved  along  the  spectrum, 
and  a  position  is  arrived  at  where 
the  last  glimmer  of  light  disap- 
pears. 

The  mirrors  A  and  B  may  both  = 

be  of  plain  glass  blackened  with  smoke  on  one  side, 
or  one  may  be  plain  glass  and  one  silvered,  or  they 
both  may  be  silvered.  This,  with  the  power  possessed 
of  altering  the  aperture  of  the  slit  of  collimator,  puts  us 
in  possession  of  ample  means  of  making  our  measures. 
We  may  also  use  the  ground-glass  arrangement  and  use 
different  diaphragms,  which  puts  a  further  power  of 
variation  in  our  hands.  I  may  at  once  state  that  the 
resulting  measurements  fell  on  the  curves,  obtained  by 
measurements  made  with  the  rotating  sectors,  a  sufficient 
proof  that  the  sectors  may  be  used  with  confidence. 
There  is  still  another  method  which    avoids   a   resort 


114  Colour   Vision, 

to  the  sectors.  A  tapering  wedge  of  black  glass  can 
be  moved  in  front  of  the  colour  slit,  and  a  different 
thickness  of  glass  will  be  required  to  cause  the  extinc- 
tion of  each  colour.  Eecently  I  have  modified  the 
extinction  box,  more  particularly  for  the  purpose  of 
using  it  where  the  spectrum  is  to  be  formed  of  a 
feeble  light,  such  as  that  of  an  incandescent  lamp  or 
a  candle.  If  a  really  black  wedge  could  be  obtained, 
this  would  seem  to  be  the  best  method,  but  no 
glass  is  really  black.  We  have,  therefore,  to  make 
a  preliminary  study  of  the  wedge  to  ascertain  ac- 
curately the  absorption  co-efficients  for  the  different 
rays,  a  piece  of  work  which  requires  a  good  deal  of 
patience,  but  which,  when  done,  is  always  at  com- 
mand. 

In  Fig.  28  two  branches  of  the  curves  are  given 
at  the  blue  end  of  the  spectrum  ;  one  is  shown  as 
the  extinction  for  the  centre  of  the  eye,  and  the 
other  of  the  whole  eye.  Of  course  the  former  observa- 
tions were  made  by  looking  direct  at  the  spot.  This 
may  appear  a  very  easy  matter,  but  it  is  not  really 
so  simple  as  it  sounds.  It  is  curious  how  little  control 
there  is  over  the  absolute  direction  of  the  eyes  when  the 
light  has  almost  disappeared.  The  axes  of  the  eyes 
are  often  directed  to  quite  a  different  point.     When 


Extinction  of  Equal  Luminosities.  1 1 5 

the  extinction  for  the  whole  eye  is  made,  the  readings 
are  really  much  easier,  as  then  the  eye  roams  where 
it  likes,  and  a  final  disg^ppearance  is  noted.  When 
the  eye  has  once  been  invested  with  a  roving  commis- 
sion, it  is  hard  to  control  it.  In  making  these  observa- 
tions it  was  therefore  advisable  to  have  data  for  the  first 
branch  of  the  curve,  before  commencing  to  observe  for 
the  later.  The  main  cause  of  difi'erence  between  the  two 
branches  of  the  curve  is  due  to  the  absorption  by 
the  yellow  spot. 

It  might  be  thought  that  with  the  curves  (Fig.  28) 
before  us,  we  have  learnt  all  we  can  regarding  the 
extinction  of  light,  but  is  it  so  ?  Surely  we  ought 
to  know  something  as  to  the  reduction  necessary  for 
extinction  of  the  difi'erent  parts  of  the  spectrum 
when  they  are  all  of  equal  luminosities  and  of 
ordinary  brightness. 

We  arrive  at  this  by  simple  calculation.  Supposing 
we  have  two  luminosities,  one  double  the  other,  it  does 
not  require  much  thought  to  find  out  that  you  have 
to  reduce  the  greater  luminosity  twice  as  much  as  the 
other  in  order  for  it  to  be  just  extinguished.  In  other  * 
words,  if  we  multiply  the  extinction  by  the  luminosity, 
we  get  what  we  want.  Now,  in  the  curves  before  us. 
we  have  taken  the  luminosity  of  the  yellow  light  near 

X   2 


ii6  Colour  Vision, 

D  as  one  amyl-acetate  lamp,  and  that  lias  a  height  in 
the  curve  showing  the  spectrum  luminosity  very  closely 
approaching  100.  We  may,  therefore,  multiply  the 
extinctions  of  a  ray  by  the  value  of  its  ordinate 
in  the  luminosity  curve  and  divide  the  result  by 
100,  and  this  will  give  us  the  extinction  of  each 
colour,  supposing  it  had  the  luminosity  of  an  amyl- 
acetate  lamp.  A  portion  of  the  curve  so  calculated  is 
shown  in  the  same  diagram  (Fig.  28)  as  a  dotted  line. 
It  appears  at  the  violet  end  as  an  approximately 
horizontal  line,  and  then  starts  rapidly  upwards,  and 
would,  if  carried  on  to  the  same  scale,  reach  far  out 
of  the  diagram  ;  but  at  the  extreme  red  it  would  be 
found  to  bend  and  again  become  horizontal.  I  would 
have  you  notice  that  the  same  is  true  not  only  for 
the  extinction  observed  with  the  centre  of  the  eye 
through  the  yellow  spot,  but  also  for  the  whole  eye. 
Such  straight,  horizontal  parts  of  the  curve  must  mean 
something. 

In  the  diagram  (Fig.  16)  of  colour  sensations  we 
see  that  in  each  of  these  two  regions  there  is  but  one 
sensation  excited,  viz.  the  violet  and  the  red.  Now, 
if  these  sensation  curves  mean  anything,  the  reduction 
necessary  to  produce  the  extinction  of  the  same  sensa- 
tion when  equally  stimulated  should  prove  to  be  the 


Fig.    30. 


ii8  Colotir  Vision, 

same,  for  there  is  no  reason  to  the  contrary,  but  exactly 
the  reverse.  Primd  facie,  then,  taking  the  Young 
theory  as  correct,  we  may  suppose  that  these  horizontal 
parts  are  due  to  the  extinction  of  one  sensation.  Let 
us  treat  it  as  such,  and  go  back  to  the  original  extinc- 
tion curve  shown  in  the  continuous  lines.  The  parts 
of  the  curve  which  lie  over  the  fairly  horizontal  dotted 
line,  at  all  events,  should  be  the  extinction  curve 
of  the  same  sensation,  but  more  or  less  stimulated  or 
excited.  As  before  explained,  if  we  have  double  the 
stimulation  at  one  part  of  the  spectrum  to  that  we 
have  at  another,  the  reduction  of  the  greater  luminosity 
to  give  extinction  will  be  double  that  of  the  lesser. 
If,  then,  we  take  the  reciprocals  of  the  extinction,  it 
ought  to  give  us  a  curve  which  is  of  the  form  of 
some  colour  sensation ;  and  when  we  arrive  at 
the  maximum,  we  may  for  convenience  make  that 
ordinate  100,  and  reduce  the  other  ordinates  pro- 
portionally. This  has  been  done  in  Fig.  30  in  the 
curves  C  and  D.  For  the  sake  of  a  name  my  col- 
league and  myself  have  named  such  curves  ''  persis- 
tency curves."  Perhaps  some  other  name  might  be 
more  fitting ;  but  still  a  poor  name  is  better  than 
none  at  all. 

When  the  persistency  curve  was  scrutinized   to  see 


Persistency  Curves,  119 

what  might  be  taken  as  its  full  signification,  I 
must  confess  that  the  result  astonished  us  some- 
what, though  we  ought  not  to  have  been  surprised. 
The  persistency  curve  C,  when  applied  (in  a  Euclidean 
sense)  to  the  curve  of  luminosity  recorded  for  the 
men  who  had  monochromatic  vision,  almost  exactly 
coincided  with  it.  In  other  words,  by  far  the 
largest  part  of  the  extinction  was  due  to  the  extinc- 
tion of  the  sensation  which  in  the  monochromatic 
vision  was  alone  excited.  If  this  be  not  the  case, 
there  is  something  in  colour  vision  which  no  theory 
which  I  am  acquainted  with  can  account  for.  Then, 
again,  the  persistency  curve  agrees  with  the  curve 
of  luminosity  when  the  intensity  of  the  spectrum  is 
very  feeble,  which  is  another  coincidence  of  a  remark- 
able character  which  some  theory  should  explain. 
[Fig.  30  gives,  besides  the  persistency  curves,  the 
luminosity  curves  of  the  normal  eye,  of  monochro- 
matic vision,  and  of  the  violet-blind;  and  an  ex- 
aggerated curve  of  the  difference  between  the  normal 
luminosity  curve  and  that  of  the  violet-blind,  and 
others  which  I  think  will  be  found  useful  for  general 
reference.] 

What  sensation  is  it   that  is  last  extinguished,  and 
which  is  possessed  by  a  certain  class  of  colour  vision  ? 


I20  Colour   Vision. 

In  the  Young  theory  it  can  only  be  the  violet  sen- 
sation. It  is  certainly  not  the  green,  and  much  less 
the  red.  It  does  not  correspond,  however,  very  well 
with  the  violet  sensation  shown  in  Fig.  16,  but  more 
with  one  which  should  be  in  the  blue. 

In  making  the  extinctions  of  light,  it  is  quite 
necessary  that  certain  precautions  should  be  taken 
to  avoid  error.  All  my  audience  know  that  when 
going  from  bright  daylight  into  a  cellar,  in  which 
only  a  glimmer  of  light  is  admitted,  but  little  can 
be  seen  at  first,  but  that,  as  the  eye  "  gets  accus- 
tomed "  to  the  darkness,  the  surroundings  will  begin 
to  be  seen,  and  after  several  minutes  what  before 
was  blackness  comes  to  be  invested  with  form  and 
detail.  So  it  is  with  the  extinction  of  light  in  the 
apparatus  described.  Observations  carried  on  before 
the  full  sensibility  of  the  eye  is  attained  are  of  no 
value.  A  recorded  set  of  observations  will  show 
this.  A  light  of  a  certain  character  was  thrown 
on  the  extinction  box,  to  be  extinguished,  and  the 
observer  entered  the  darkened  room  from  the  full 
glare  of  daylight.  The  eye  was  placed  at  the  eye 
end  and  kept  there,  and  the  extinctions  were  made 
one  after  the  other  till  they  became  very  fairly  con- 
stant.    The  following  is  the  result  : — 


Proo^ressive  Sensitiveness, 


121 


Times  of  Observation. 

Readings. 

At  the  commencement         .          .          .          .          1*0 

After                38  sec. 

3-2 

After                 53  sec. 

4-9 

After    1  min.  11  sec. 

6-9 

After    1  min.  44  sec. 

10-5 

After    2  min.  43  sec. 

17-0 

After    3  min.  44  sec. 

27-5 

After    4  min.  52  sec. 

43-0 

After    5  min.  59  sec. 

63-0 

After    6  min.  41  sec. 

78-0 

After    7  min.  28  sec. 

89-0 

After    8  min.  32  sec. 

96-0 

After  10  min.  46  sec. 

103-0 

After  12  min. 

103-0 

(For  convenience  the  first  reading  is  unity ;  the  other  numbers  arc 
the  inverse  of  the  extinction  value.) 

The  eye  apparently,  under  the  conditions  in  which 
these  observations  were  made,  was  at  least  100  times 
more  sensitive  to  very  faint  light  after  twelve  minutes 
than  it  was  at  the  beginning,  and  that  then  concordant 
readings  could  be  made.     It  will  now  be  quite  under- 

I stood  that  before  any  serious  measures  can  be  made 
this  intei*val  must  elapse,  and  also  that  the  light, 
finding  its  way  to  the  end  of  the  box  to  illuminate 
the  spot,  should  never  be   strong,  otherwise  the    eye 


(     122    ) 


CHAPTER    X. 

Before  considering  the  subject  of  the  extinction  of 
light  by  other  types  of  colour  vision,  attention  must 
be  called  to  what  has  already  been  brought  before  you. 
The  various  colours  of  the  spectrum  have  to  be  reduced 
to  the  following  amounts  before  they  sufifer  extinction, 
the  orange  light  at  D  being  of  the  value  of  one  candle. 
(See  appendix,  page  217,  for  complete  tables.) 

Reduction  in  ■d^«,„»i« 

MiUionths.  Remarks. 

B 10,000         or       yig^      approximately    pure     red 

sensation 

C     1,100         or       7^^       rather  more  scarlet 

D    50         or     32^0^0-    orange  light 

E    6*5     or    x5¥Voi5"    a  green  chosen  by  Maxwell 

as  a  standard  colour 

F    15*0     or     ^tooo     beginning  of  the  blue 

Blue    Lithium  85*0     or     ttt^o^     a  good  sample  of  blue 

G    300*0     or      3-300      approximately  pure  sensa- 

tion of  violet. 

If  we  make  these  same  colours  all  of  the  luminosity 
of  one  amyl-acetate  lamp  ( •  8  of  a  candle),  we  find 
that  the  numbers  are  as  follows  : — 


Visibility  of  Illumination,  123 


Keduction  in 
Millionths. 

Keduction  in 
Millionths. 

B    ... 

300       1 

F 

•9 

C    ... 

225       1 

Blue  Lithium 

1-1 

D    ... 

48       , 

G 

1-1 

E    ... 

3-3 

These  numbers  are  remarkable,  and  we  may  enforce 
what  they  mean  in  this  way.  The  energy  of  radiation, 
and  of  light  also  when  of  ordinary  luminosity,  varies 
inversely  as  the  square  of  the  distance  from  an  incan- 
descent body  when  of  small  dimensions.  But  from  the 
above  it  seems  that  a  white  screen  receiving  the  rays 
from  an  amyl-acetate  lamp  in  an  otherwise  perfectly 
dark  place,  and  having  a  colour  which  stimulates  the  red 
sensation  alone,  would  be  invisible  at  58  feet  distance, 
for  there  would  not  be  enough  energy  transmitted  to 
stimulate  the  red  perceiving  apparatus  sufficiently  to 
give  the  sensation  of  light.  If  it  were  an  orange  light, 
such  as  sodium,  of  the  same  luminosity,  we  should  have 
to  move  it  from  the  screen  142  feet  before  the  same 
result  was  attained.  With  the  green  light  at  E,  the 
distance  would  be  550  feet,  and  with  the  violet  the 
distance  would  be  increased  to  1000  feet.  The  re- 
duction in  intensity  of  white  light,  which,  when  of 
ordinary  brightness,  is  warm,  would  make  it  colder,  for 
the  red  would  disappear,  and  finally  the  residue  of 
light,  just  before  extinction,  would  become  a  cold  grey, 


124  Colottr   Vision. 

due  to  tlie  absence  of  all  colour.  The  changes  in  hue 
that  would  occur  are  variable,  the  variation  being  due 
to  the  loss  of  colour  of  the  different  rays  for  different 
amounts  of  reduction,  and  then  their  final  extinction. 
We  can  place  two  patches  of  white  light  on  the  screen, 
and  gradually  reduce  one  in  intensity,  keeping  the  other 
of  its  original  value.  No  one  would  expect  that  the 
two  would  be  dissimilar  in  hue,  as  they  appear  to  be 
when  the  former  is  moderately  near  the  extinction  value. 
If  we  wish  to  see  this  perfectly,  we  should  use  an 
extinction  box  and  view  it  away  from  the  surroundings, 
which  must  be  more  or  less  slightly  illuminated. 

It  has  already  been  stated  that  the  persistency  curve 
for  persons  who  have  normal  colour  vision  is  closely 
the  same  as  that  recorded  for  those  who  are  of  the 
monochromatic  type.  As  this  is  so,  we  must  expect 
to  find  that  the  persistency  curve  of  these  last  is  the 
same  as  their  luminosity  curve.  We  put  this  to  the 
test  of  experiment  and  found  that  our  reasoning 
was  correct,  for  the  persistency  curve  could  be  almost 
exactly  fitted  to  it.  (See  table,  pages  217  and  222.) 
The  slight  difference  between  them  can  be  credited  to 
the  fact  that  the  whole  eye  may  have  been  brought 
into  use  during  the  extinction  observations,  the  centre 
of  the   eye  not  being  exclusively  used.     The  Figure  31 


Monochromatic   Vision. 


125 


shows  both  the  extmction  and  the  persistency  curves, 
and  also  the  curve  of  luminosity  for  the  normal  eye. 


Fig.  31. 


126  Colour   Vision, 

The  former  were  derived  from  a  case  P.  sent  for 
examination.  P.  and  Q.  are  brothers,  each  of  whom 
possesses  but  one  colour  sensation,  and  examination 
showed  that  their  vision  was  identical.  Mr.  Nettleship 
has  kindly  given  me  the  following  particulars  regarding 
them  : — "  Their  acutes  of  vision  (form  vision)  in  ordinary- 
daylight  is  only  one-tenth  of  the  normal.  A  younger 
sister  and  brother  are  idiotic  and  almost  totally  blind, 
and  in  one  of  these  the  optic  nerves  show  clear  evidence 
of  disease.  Hence,  the  colour  blindness  of  P.  and  Q. 
must  almost  without  doubt  be  considered  as  the  result 
of  disease,  perhaps  ante-natal,  involving  some  portion 
of  the  visual  apparatus."  A  lack  of  acuteness  of 
vision  would  be  expected  from  the  small  amount 
of  light  they  perceive  compared  with  normal  vision. 
The  fact  that  two  of  a  family,  not  twins,  possess 
exactly  the  same  colour  sense,  and  that  their 
extinction  curves  are  entirely  different  to  those 
suffering  from  post-natal  disease,  but  similar  to 
those  of  normal  vision,  point  to  their  colour  blind- 
ness as  falling  in  the  same  general  category  as 
that  of  the  congenital  type.  To  this  I  shall  refer 
again. 

We  may  reason  still  further.  With  the  red-  and 
green-blind  the  violet  sensation  is  still  present,  and  we 


I 


Extinction  by  the  Colour  Blind,  127 

may  therefore  expect  that  their  extinction  curves,  and 
consequently  their  persistency  curves,  should  be  alike, 
and  should  also  agree  with  that  made  from  your 
lecturer's  observations.  A  study  of  Figures  32  and  33 
will  tell  you  that  such  is  practically  the  case.  The 
former  shows  the  luminosity,  the  persistency,  and  the 
extinction  curves  of  a  completely  red-blind  subject,  and 
the  latter  the  same  curves  for  a  green-blind  subject  (see 
pages  223  and  224).  Both  were  excellent  observers,  and 
their  examination  was  easy,  owing  to  the  acquaintance 
with  scientific  methods.  The  accuracy  of  their  results 
may  be  taken  as  unquestionable.  Each  of  them  may  be 
taken  as  a  representative  of  their  own  particular  type  of 
colour  blindness.  There  is  an  agreement  between  them 
at  the  violet  end,  but  a  deviation  at  the  red  end  of  the 
spectrum.  The  general  form  of  the  curves  indicates 
that  the  same  sensation  is  extinguished  last  in  all. 
Now,  have  we  any  other  criterion  to  offer  ?  We  have. 
In  the  first  instance,  we  have  the  violet-blind  person 
to  compare  with  the  others,  and  also  another  observer 
who  had  monochromatic  vision,  but  whose  sensation 
was  different  to  that  of  the  two  monochromatic  cases  we 
f  have  so  far  brought  to  your  notice.  We  have  already 
stated  the  peculiarities  in  colour  nomenclature  of  the 
violet-blind  case.    His  curve  of  luminosity  for  the  spec- 


-^-^-^p^hAi 


^  WH^m^i^ 


•m^^ 


128  Colour   Vision. 

trum  was  taken  (page  227),  and  when  compared  with  the 
curve  of  normal  luminosity,  it  became  evident  that  in 

Fig.  32. 

m 

■■■■■■■■■■■li   I  I 

■■■■■■■■■■■■■■■Bai 


Y/pWd 


Violet-blindness.  129 

the  red  and  up  to  the  orange  his  measures  were  those 
which  a  normal  eye  would  make ;  but  that  the  lumi- 
nosity fell  off  in  the  green,  and  finally  disappeared 
to  an  immeasurable  quantity  in  the  violet  (see  Fig. 
30,  curves  M  and  F).  If  his  measures  of  spectrum 
luminosity  are  deducted  from  those  of  the  normal  eye, 
and  the  ordinates  be  increased  proportionately  to 
make  the  maximum  difference  100,  the  figure  so 
produced,  when  compared  with  the  luminosity  curve 
obtained  from  the  monochromatic  observers,  was  found 
to  be  the  same,  and  consequently  with  the  persistency 
curves  above  referred  to.  Endeavours  were  made  to 
gain  a  good  extinction  curve,  but  the  results  were  not 
as  successful  as  could  be  desired ;  but  it  was  ascertained 
that,  without  doubt,  his  most  persistent  sensation  was 
not  more  than  -p|-g-  as  lasting  as  that  of  the  normal 
eye,  or  to  put  it  in  another  way,  his  green  at  E  was  only 
extinguished  when  the  energy  falling  on  his  eye  was 
180  times  greater  than  that  at  which  it  vanished  with 
the  normal  eye.  This  plainly  teaches  us  that  the 
missing   sensation   was   that   which,   when  present,    is 

I  ordinarily  the  most  persistent. 
The  next  is  a  case  of  monochromatic  vision,  which 
differs  from  those  previously  brought  before  you,  and  I 
cannot  do  better  than  describe  it  in  the  words  which 


150  Colour  Vision, 

General  Festing  and  myself  employed  in  our  paper  in 
the  "  Philosophical  Transactions." 

Fig.  33. 


Green  Monochromatic   Vision.  131 

The  patient  (B.  C.)  had  been  examined  by  Mr. 
Nettleship,  who  kindly  secured  his  attendance  at  South 
Kensington  for  the  purpose  of  being  examined  by 
the  spectrum  and  other  tests.  [Mr.  Nettleship  states 
that  this  case  is  without  doubt  a  genuine  case  of 
congenital  colour  blindness,  without  any  trace  what- 
ever of  disease.]  B.  C.  is  a  youth  of  19,  who  has 
served  as  an  apprentice  at  sea.  His  form  vision  is 
perfect,  and  he  is  not  night  blind.  He  can  see  well 
at  all  times,  though  he  states  that  on  a  cloudy  day 
his  vision  seemed  to  be  slightly  more  acute  than  in 
sunshine.  He  was  first  requested  to  make  matches 
with  the  Holmgren  wools  in  the  usual  manner,  with 
the  result  that  he  was  found  to  possess  monochromatic 
vision.  He  matched  reds,  greens,  blues,  dark  yellows, 
browns,  greys,  and  purples  together ;  and  it  was  a  mat- 
ter of  chance  if  he  selected  any  proper  match  for  any  of 
the  test  colours.  Finally,  when  pressed,  he  admitted 
that  the  whole  of  the  heap  of  wools  were  "blue"  to  him, 
any  one  only  differing  from  another  in  brightness.  The 
brighter  colours  he  called  "dirty"  or  "pale"  blue,  terms 
which  eventually  proved  to  be  synonymous.  We  then 
examined  him  with  patches  of  monochromatic  spectrum 
colours  by  means  of  the  colour  patch  apparatus.  He  de- 
signated every  colour  as  "blue,"  except  a  bright  yellow, 

K  2 


132  Colour   Vision. 

which  he  called  white,  but  when  the  luminosity  of  this 
colour  was  reduced  he  pronounced  it  a  good  blue.  So  with 
white,  as  the  illumination  was  decreased,  he  pronounced 
it  to  pass  first  into  dirty  blue,  and  then  into  a  full  blue. 

Colour  discs  were  then  brought  into  requisition, 
and  it  was  hard  at  first  to  know  how  to  make  the 
necessary  alterations,  owing  to  the  terms  he  employed 
to  express  the  difierence  which  existed  between  the  inner 
disc  and  the  outer  grey  ring.  By  noting  that  a  pale 
"blue"  passed  into  a  pure  blue  when  the  amount  of  white 
in  the  outer  ring  was  diminished,  and  that  the  inner  disc 
was  described  as  "pale"  or  "dirty"  when  the  outer  ring 
was  described  as  "a  very  full  blue,"  we  were  enabled  to 
make  him  match  accurately  a  red,  a  green,  and  a  blue 
disc  separately  with  mixtures  of  black  and  white. 

The  following  are  the  equations  : — 

360  red      =315  black  +  45  white. 
360  green  =  258  black  +  102  white. 
360  blue    =  305  black  +  55  white. 

With  these  proportions  he  emphatically  stated  that 
all  were  good  blues,  and  that  the  inner  disc  and  outer 
ring  were  identical  in  brightness  and  in  colour. 

It  may  be  remarked  that  this  is  a  case  of  congenital 
colour  blindness,  and  that  there  is  reason  to  believe  that 
some  of  his  ancestors  were  colour  blind. 


Gi^een  Monochromatic   Vision,  133 

Before  using  the  discs  an  attempt  was  made  to  ascer- 
tain the  luminosity  of  the  spectrum  as  it  appeared  to 
him.  His  readings,  however,  were  so  erratic  that 
nothing  could  be  made  out  from  these  first  observations, 
except  to  ^n  the  place  of  maximum  luminosity,  the 
terms  "  pale  "  and  "  dirty  "  puzzling  us  as  to  their  real 
meanings.  After  the  experience  with  the  discs  we  had 
a  clue  as  to  what  he  wished  to  express  by  pale  or  dirty 
blue,  which  only  meant  that  the  colour  or  white  was 
too  bright,  and  on  making  a  second  attempt  he  matched 
the  luminosities  of  the  two  shadows  as  easily  as  did  P. 
and  Q.,  the  other  cases  of  monochromatic  vision.  The 
method  adopted  was  to  diminish  the  white  light  illu- 
minating one  shadow  to  the  point  at  which  he  pro- 
nounced it  a  good  blue,  when  a  slight  alteration  in  the 
intensity  was  always  sufficient  to  secure  to  his  eye 
equality  of  luminosity  between  it  and  the  coloured 
shadow  without  his  perceiving  any  alteration  in  the 
saturation. 

The  curve  of  luminosity.  Fig.  34,  is  a  very  remarkable 
one,  being  different  in  character  to  that  of  P.  and  Q.,  the 
maximum  being  well  on  the  D  side  of  E.  A  great  falling 
off  in  the  luminosity  when  compared  with  that  measured 
by  the  normal  eye  will  be  noticed  both  in  the  blue  and 


134 


Colour  Vision. 


was  therefore  presumptive  that  B.  C.'s  colour  sensation 
was  neither  red  nor  blue,  but  probably  a  green. 

Fig.  84. 


I 

■I 
■ 

I 


■■■■■■■■■■I 


■■■■■■ 
■■■■■n 

■■■■■RSh 

iUHIi 


wmummk 

■■■agnnnBassanisaiBia 
ill 


sanisaBH 

—a 


■BHHl 


B.  C.'s  Lumiuosity  and  Extiuction  Curves. 


Extinction  of  Light  b)^B^£:^ ^^i^^^ 

The   next   test   was   made   to   tlirb^yii^%16^>;  TO 


V  - 


point.  He  made  observations  of  the  extmcticMi  of  th^ 
different  parts  of  the  spectrum.  His  observatieiia- 
were  very  fair,  except  on  the  violet  side  of  F,  where 
they  became  slightly  erratic,  but  by  requesting  him  to 
use  all  parts  of  his  retina  to  obtain  the  last  glimpse  of 
light,  a  very  concordant  curve  resulted,  as  shown  in  Fig. 
34.  Some  of  his  observations  at  this  part  were 
evidently  made  with  the  centre  of  the  retina,  for  they 
gave  readings  which,  when  the  "  persistency "  curve 
was  calculated,  and  these  observations  treated  as  part  of 
the  extinction,  agreed  with  the  luminosity  curve.  We 
may,  therefore,  conclude  that  B.  C.  has  a  region  in  the 
retina  in  which  there  is  an  absorbing  medium  corre- 
sponding to  the  yellow  spot  of  the  normal  eyed.  This 
is  diagrammatically  shown  in  Fig.  34  by  the  difference 
in  height  of  ordinates  in  the  persistency  (dotted)  and 
the  luminosity  curves.  On  the  red  side  of  the  maximum 
the  two  curves  are  practically  identical,  except  from 
Scale  number  54.  At  this  point  it  is  probable  that  the 
white  light  which  illuminated  the  prism  vitiated  the 
readings  to  some  degree.  At  the  violet  end  something 
similar,  doubtless,  occurs,  but  it  is  masked  by  the 
difference  that  exists  in  the  extinction  by  the  central 
part  of  the  retina  and  that  of  the  whole  eye. 


136  Colour  Vision, 

It  must,  however,  be  remarked  that  the  amount  of 
reduction  of  the  intensity  of  a  ray  to  produce  extinction 
is  very  different  for  B.  C.  and  for  the  normal  eyed,  or 
for  the  red-  and  green-blind  or  for  P.  and  Q.  B.  C. 
can  bear  nearly  200  times  less  reduction  for  the  rays 
near  E.  We  have  already  pointed  out  that  the  same  is 
practically  the  case  with  M.,  whom  we  presume  to  be 
violet-blind.  We  may  therefore  deduce  the  fact  that 
the  monochromatic  vision  in  this  case  is  of  a  totally 
different  type  to  that  of  P.  and  Q.,  and  that  the  last 
sensation  to  be  lost  is  the  same  as  that  of  M.  If  any 
violet  sensation  were  present  in  either,  the  fact  would 
be  made  evident  by  the  order  of  the  extinction.  The 
sensation  of  B.  C.  is  thus  apparently  the  green  sensation, 
though  that  this  particular  sensation  is  exactly  the  same 
as  that  absent  in  the  green-blind  is  not  certain. 

The  observations  made  by  the  different  types  of  the 
colour  blind  seem  to  me  to  throw  great  light  on  the 
theory  of  colour  vision.  They  show  that  when  the 
violet  sensation  is  present,  according  to  the  Young 
theory,  the  extinction  shows  its  presence  ;  and  that 
where  this  sensation  is  absent,  the  reduction  of  light 
necessary  to  produce  extinction  is  greatly  less,  and 
may  with  great  certainty  be  attributed  to  a  different 
sensation  being  the  final  one  to  disappear. 


(  ^37  ) 


CHAPTEK    XL 

I  HAVE  SO  far  spoken  only  of  normal,  or  physiological, 
colour  blindness ;  a  peculiarity,  or  defect,  present  at 
birth,  and,  as  far  as  is  at  present  known,  irremediable, 
but  not  associated  with  any  defect  of  the  visual  func- 
tions, or  with  any  disease  or  any  optical  peculiarities. 
What  the  nature  and  seat  of  this  defect  may  be — 
whether  in  the  eye  or  in  the  sensorium — is  at  present 
unknown,  although  some  of  the  characteristics  of  the 
deficiency  in  colour  sensation,  I  believe,  seem  to  indi- 
cate the  existence  of  a  special  part  of  the  brain  endowed 
with  the  functions  for  perceiving  colour. 

But  cases  are  well  known  to  medical  men  in  which 
colour  vision, .  normal  to  start  with,  fails  in  greater  or 
less  degree  in  connection  with  disease.  This  part  of 
the  subject  is  large  and  very  complex,  and  requires  for 
its  full  elucidation  an  acquaintance  with  the  diseases 
and  disorders  of  the  eye.  Many  of  the  phenomena 
accompanying  acquired  colour  blindness,  however,  are 


i^S  Colour   Vision. 

of  great  interest  to  the  physicist  in  his  study  of  colour 
vision,  more  particularly  in  regard  to  the  test  of  the 
truth  of  any  particular  theory.  Through  the  kindness 
of  several  medical  men,  and  Mr.  Nettleship  in  par- 
ticular, I  have  had  the  opportunity  of  examining  by 
the  colour  apparatus  several  types  of  colour  blindness 
due  to  disease.  One  feature,  common,  I  understand, 
to  all,  or  nearly  all,  cases,  is  the  presence  of  some 
disease  of  the  optic  nerve.  Defective  sight — from  loss 
of  transparency  of  the  cornea,  the  crystalline  lens,  or 
other  transparent  parts  of  the  eye — does  not  interfere 
with  the  perception  of  colour ;  nor  is  true  colour 
blindness,  as  I  am  informed,  well  marked,  if  present 
at  all,  in  disease  limited  to  the  choroid  and  retina  (see 
Fig.  1).  Even  in  cases  of  the  disease  of  the  optic 
nerve,  medical  authorities  tell  us  that  great  diifferences 
exist  in  the  amount  of  colour  defect,  and  that  although 
the  colour  defect  always  goes  along  with  some  other  serious 
visual  loss,  either  of  form,  light,  or  field,  the  relation 
between  these  several  factors  of  the  visual  defect  is  by 
no  means  always  the  same,  so  far  as  can  be  judged  by 
the  tests  commonly  used  by  ophthalmic  surgeons.  They 
tell  us  that  in  some  cases  of  disease  of  the  optic  nerve, 
colour  vision  when  tested  by  the  wool  test,  which  will 
be  described  shortly,  may  be  almost  perfect,  whilst  the 


Tobacco  Amblyopia.  139 

capacity  for  reading  test  letters  of  the  alphabet  may  be 
extremely  bad,  and  vice  versa.  It  seems  that  in  some 
cases  these  discrepancies  cannot  be  accounted  for ;  but 
in  others  the  facts  can  be  explained  by  the  limitation 
of  the  disease  to -certain  fibres  of  the  optic  nerve. 
Thus,  if  those  fibres  which  supply  the  yellow  spot 
region  of  the  retina  are  alone  involved,  direct,  or 
central,  vision  will  be  much  damaged  both  for  form  and 
colour,  whilst  a  little  further  from  the  centre  of  the 
field,  the  visual  functions  in  such  a  case  are  often  quite 
normal.  From  what  has  been  said  in  the  opening 
chapters,  this  will  ibe  understood  to  be  that  the  colour 
vision  is  perfect,  but  the  definition  of  form  more  or  less 
imperfect.  We  are  told  that  cases  of  this  type  have 
long  been  known  and  are  comparatively  common,  and 
often  favourable  as  regards  recovery  ;  that  the  mischief 
may  affect  one  optic  nerve,  or  both  ;  that  when  both 
are  diseased  the  malady  is  usually  due  to  the  action  of 
some  toxic  substance,  and  that  of  all  substances  known  to 
have  this  particular  effect  on  the  optic  nerves  tobacco 
is  the  most  important.  I  dwell  a  little  on  this  variety 
— damage  to  form  and  colour  sense  at  the  centre  of  the* 
visual   field   of    each   eye   from   limited,    and   usually 

t curable,  disease  of  the  optic  nerve — on  account  of  its 
interest  to  myself  in  the  investigations  I  have  made, 
I 


140  Colour   Vision, 

and  also  on  account  of  the  degree  of  practical  impor- 
tance which  it  assumes  in  connection  with  the  proper 
reading  of  signals  and  coloured  lights.  These  cases 
of  "  tobacco  amblyopia,"  as  it  is  pathologically  called, 
are,  of  course,  always  found  in  men;  and  it  may 
occasionally  happen  that  such  a  man,  if  an  engine 
driver,  signalman,  or  a  look-out  man  on  board  ship, 
may  still  see  form  sufficiently  well  to  see  his  signals, 
but  may  mistake  their  true  colours.  From  evidence 
given  before  the  Committee  of  the  Royal  Society  on 
Colour  Vision,  it  appears  that  the  disease  causing  this 
type  of  colour  blindness  is  usually  produced  by  the 
over-use  of  tobacco,  aided  by  mental  depression  and  a 
low  state  of  health.  As  we  have  no  sumptuary  laws, 
cases  of  tobacco  blindness  must  frequently  occur,  and 
it  should  be  the  care  of  all  who  have  the  management 
of  railways  or  shipping  to  take  measures  for  preventing 
persons  suffering  from  this  disease  from  occupying 
posts  which  require  perfect  colour  vision  in  order  to 
prevent  the  possibility  of  loss  of  life. 

Congenital  colour  blindness  can  at  once  be  discovered, 
and  its  possessor  be  excluded  from  any  post  in  which 
normal  colour  perception  is  necessary,  but  with  this 
type  a  single  examination  is  no  safeguard,  as  it  may 
be  developed  at  any  period  of  a  man's  career.     The 


Central  Scotoma.  141 

disease  is,  I  believe,  a  progressive  one,  and  at  first  is 
most  generally  unrecognised,  the  deficiencies  of  vision 
being  usually  slight  at  its  commencement.  It  is 
very  often  brought  to  the  notice  of  the  sufferer  by 
finding  he  is  unable  to  read.  The  words  at  first 
seem  only  slightly  indistinct,  but  later  become  un- 
decipherable, and  as  time  goes  on  he  is  unable  to 
even  see  the  letters.  He  or  his  friends  then  usually 
think  it  time  to  consult  the  specialist.  In  tobacco 
amblyopia  the  area  of  insensibility  is  central,  and  it 
may  subtend  a  very  small  angle  or  one  which 
covers  a  considerable  portion  of  the  field.  I  am  not 
aware  that  it  ever  extends  over  it  all,  but  it  very 
generally  covers  the  yellow  spot.  Now  as  the  eye 
naturally  receives  the  image  on  the  centre  of  the 
retina,  it  follows  that,  as  the  ability  to  distinguish 
some  colours  is  absent  in  that  particular  region,  the 
patient  is  practically  colour  blind,  though  he  can  dis- 
tinguish them  on  most  parts  of  the  retina  which  are  not 
affected.  As  regards  form  vision,  it  was  mentioned 
in  the  first  chapter  that  in  a  healthy  eye  it  is  much 
more  acute  at  the  centre  than  towards  the  periphery, 
and    instances    were    given   of   the    angular   distances 

I  apart  that  black  dots  on  a  white  ground  were  required 


142  Colour  Vision, 

objects  when  the  images  were  received  on  the  centre 
of  the  retina,  and  at  the  periphery.  Sharp  definition 
may  be  said  to  be  almost  confined  to  3°  of  angular 
distance  at  the  centre,  and  most  probably  this  is  a 
happy  state  of  affairs,  for  if  we  could  see  equally 
distinctly  with  the  whole  field  of  vision,  the  mind 
would  be  distracted  from  the  object  which  it  wished 
primarily  to  contemplate. 

Bearing  in  mind  the  want  of  definition  beyond  3°, 
and  the  indistinctness  caused  by  a  diseased  central 
area,  it  will  not  be  surprising  to  find  that  form  vision  in 
these  cases  is  imperfect  throughout,  though  the  colour 
perception  outside  such  area  may  be  unimpaired.  But, 
practically,  men  suff'ering  from  this  disease  are  colour 
blind  to  coloured  objects,  such  as  a  signal  light  on  a 
railway  or  a  ship's  light  at  sea.  They  may  see  that 
there  is  light  at  the  distant  signal  or  on  the  bow  of  a 
vessel,  but  will  be  unable  to  interpret  correctly  the 
colour.  The  colours  which  fail  to  make  visual  impres- 
sions are  the  reds  and  greens.  Some  will  distinguish 
yellow,  and  very  nearly  all  will  distinguish  blue  with 
the  centre  of  the  eye.  If  a  bright  spectrum  be  thrown 
on  the  screen,  and  a  tobacco-blind  person  be  requested 
to  name  the  colours  of  the  different  parts  pointed  out  to 
him,  it  is  often  the  case  that  as  his  eyes  follow  the 


Colour  Fields  of  the   Tobacco  Blind.  1 43 

pointer  lie  will  tell  you  that  in  the  extreme  red  he 
sees  no  light,  but  in  the  bright  red  he  sees  dull  white. 
The  bright  yellow  he  will  tell  you  is  a  pale  yellow 
or  white,  according  as  his  case  is  a  moderate  or  bad 
one ;  the  green  he  will  call  white,  and  the  blue  and 
violet  he  will  designate  correctly.  At  the  same  time 
that  his  eye  is  turned  away  to  another  colour,  he 
will  see  the  true  colour  of  the  part  of  the  spectrum 
which  he  has  just  incorrectly  named,  but  it  will  dis- 
appear again  as  he  turns  his  eyes  back  again.  This 
tells  us  that  his  sense  of  colour  is  apparently  unaffected 
outside  the  diseased  area. 

At  page  10,  a  description  has  been  given  of  the 
manner  in  which  the  field  for  colour  and  light  has 
been  determined,  and  if  this  same  method  be  pursued 
with  persons  suffering  from  this  form  of  colour  blind- 
ness we  get  some  remarkable  results.  Fig.  35  is  the 
chart  of  the  eye  for  red  and  for  white,  which  was 
made  by  a  case  of  tobacco  blindness.  The  yellow 
spot  is  entirely  affected,  and,  as  is  very  common, 
it  extends  to  the  blind  spot  in  the  eye.  At  no  place 
within  that  area  can  red  be  seen,  though  blue  is  imme- 
diately recognised.  The  extent  of  the  field  for  white 
tis  that  found  under  normal  conditions,  and  except  for 
the  diseased  area  the  same  is  true  for  the  red.     The 


144 


Colour  Vision, 


fields  for  both  eyes  are  given :  that  for  the  left  eye  in 
the  left-hand  chart,  and  that  for  the  right  eye  in  the 
right-hand  chart.     The  small  dark  spots  within  the  5° 

area  are  places 
where  the 
colour  sensa- 
tion is  most 
defective.  The 
part  in  the 
central  dark 
area  shaded 
with  lines  in 
this  direction 
////shows  the 
portion  of  the 
field  which  is 
insensitive  to 
red,  though 
not  to  light, 
whilst  the  re- 
mainder of  the 
shaded  central 
area  indicates 
the  extent  of 
the  field  w^hich 


Testing  for  Colour,  145 

is  sensitive  to  red.  The  field  for  light  generally  is  also 
shown  by  the  (approximately)  rectangular  unshaded 
area.  Although  the  area  occupied  by  the  insensitive 
part  of  the  retina  is  small  compared  with  the  whole, 
yet  it  is  in  that  part  which  is  used  for  distinct  vision. 

For  testing  for  colour  the  apparatus,  Fig.  3,  arranged 
so  that  the  patch  of  colour  has  the  white  patch  along- 
side, is  the  most  useful,  but  it  is  as  well  then  to  use 
a  surface  of  patch  about  \  inch  square  only,  and  thus 
to  confine  the  image  as  nearly  as  may  be  to  the  spot 
on  the  retina  which  is  defective.  These  cases  of  central 
scotoma  are  by  no  means  very  easy  to  test ;  for  it 
frequently  happens  that  before  they  are  able  to  dis- 
tinguish that  there  are  two  patches  side  by  side,  they 
have  to  approach  very  close  to  the  screen.  If  this  be 
the  case,  however,  it  will  usually  be  found  that  the 
patches  of  i  inch  side  are  still  efficient,  as  the  near 
approach  of  the  eyes  to  the  screen  indicates  a  wide 
area  as  being  affected,  so  that  the  image  still  lies 
within  the  diseased  retinal  area.  In  some  instances 
the  colours  named  will  vary  very  considerably ;  some- 
times, for  instance,  a  red  will  be  named  as  grey,  and 
then  immediately  after  as  pale  red.     This  is  generally  - 

fiue  to  the  diseased  area  being  small,  and  a  very  slight 
hange  in  the  direction  of  the  axis  of  the  eye  causes' 


146  Colour   Vision. 

it  to  be  seen  in  nearly  its  true  colour,  part  being 
viewed  with  the  diseased  and  part  with  the  healthy 
portion  of  the  retina.  With  the  wool  test,  which  we 
shall  describe  later,  it  is  the  commonest  thing  possible 
for  colour-blind  persons  who  have  a  central  scotoma 
to  match  accurately  the  different  test-skeins,  for  the 
reason  that  the  images  of  the  skeins  of  wool  are  so 
large  that  they  are  received  on  the  parts  of  the 
retina  which  are  not  diseased.  These  same  colours, 
however,  if  presented  to  them  in  small  patches,  will 
inevitably  show  the  defect  in  vision. 

With  this  end  in  view,  I  have  had  a  set  of  brick-clay 
pellets  some  -^-inch  in  diameter,  painted  with  water- 
colours  mixed  with  soluble  glass  solution  of  the  same 
colours  as  the  wools.  These  are  placed  in  a  shallow 
tray,  and  presented  to  patients  affected  with  this 
central  colour  blindness  to  pick  out  all  the  pellets 
which  match  reds  and  greens.  They  will  tell  you 
that  they  see  neither  one  nor  the  other,  though  they 
will  pick  out  the  blue  pellets  unerringly.  A  red  pellet 
they  will  match  with  a  red,  green,  grey,  or  a  brown 
one,  and  a  green  one  with  the  same.  If,  however, 
you  instruct  them  to  direct  their  eyes  a  few  degrees 
away  from  the  tray,  they  will  tell  you  they  see  all 
the  colours,  and  as  they  endeavour  to  pick  them  out, 


Pellet  Test.  J47 

they,  with  a  natural  instinct,  direct  their  eyes  again 
to  the  collection,  when  once  more  the  colours  vanish. 
It  is  almost  piteous  sometimes  to  see  the  distress 
which  this  simple  test  occasions.  The  sight  of  the 
colours  for  an  instant  and  their  immediate  disappear- 
ance in  the  cases  that  I  have  tried,  seem  indicative 
of  something  terrible,  for  they  usually  have  no  idea 
of  the  cause  of  this  (to  them  almost  miraculous) 
phenomenon.  I  have  seen  these  colour  blind  tested 
with  a  pair  of  ordinary  bull's-eye  lanterns,  placed  side 
by  side,  with  diaphragms  of  moderate  size  with 
coloured  glasses,  which  can  be  changed  at  will,  in 
front.  At  twelve  feet  distance  they  will  often  see  both 
lights  as  one,  but  as  they  approach  they  will  make  out 
two  lights  and  call  them  both  white,  or  sometimes 
they  will  make  a  guess  and  call  a  green  red.  or  vice 
versd.  It  goes  without  saying  that  such  eyesight  is 
useless  for  reading  signals,  and  indeed  for  any 
purpose  whatever.  Sometimes,  but  I  believe  this  is 
rare,  no  colour  whatever  can  be  distinguished. 


(   h8  ) 


CHAPTER    XII. 

I  WILL  now  give  in  full  the  result  of  the  examination 
of  a  patient  who  was  suffering  from  tobacco  blindness. 
X. ,  aged  thirty-six,  a  commercial  traveller,  was  suffering 
from  rather  severe  tobacco  amblyopia.  The  scotoma 
was  a  very  marked  one,  and  the  loss  of  colour  sensa- 
tion most  complete.  Mr.  Nettleship,  who  furnished 
the  case,  has  kindly  added  the  following  remarks  on 
the  case  : — 

His  acuteness  of  vision  was  -3%  with  the  right  eye 
and  -g^g-  with  the  left.  He  smoked  half-an-ounce  of 
"  shag  "  daily  and  drank  about  four  pints  of  beer.  His 
sight  had  been  failing  for  about  two  months=  As  is 
common  in  early  stages  of  this  disease  the  ophthalmo- 
scope revealed  no  decided  changes  at  the  optic  discs. 

He  passed  the  test  of  the  Holmgren  wools  satis- 
factorily, proving  that  the  usual  vision  was  normal  for 
colour,  but  failed  at  once  with  the  pellet  test. 

The  objects  in  view  were  to  test  his  perception  of 
the  spectrum  colours,  and  then  the  extent  of  his  retinal 


Luminosity  Curves.  149 

field  for  colour.     This  last  is    not  recorded  here.     The 
spectrum  colours  were  reduced  to  uniform  luminosity 

Fig.  36. 


150  Colotcr   Vision. 

between  X4600  and  X6600.      Diaphragms  containing 
holes  of  different  sizes  were  placed  in  front  of  the  last 

prism,  and  thus  a  round  spot  of  monochromatic  light  of 

the  same  luminosity  was  produced  upon  the  screen  when 

a  slit  was  passed  through  the  spectrum.     From  the  red 

end  to  X  5270  he  called  the  whole  of  the  colours  white, 

and  from  that  point  he  began  to  see  blue,  called  the 

colours  bluish  and  blue.     When  the  full   illumination 

for   all  the  colours  was   used,   the   same   results  were 

obtained.       From   this   examination    it   would    appear 

that  he  was  totally  deprived  of  the  sensation  of  any 

colour  except  of  blue.     A  subsequent  examination  of 

his   perception   of    the    luminosity   of    different   rays, 

however,  has  to  be  taken  into  account,  for  in  the  first 

examination  he  had  no  light  of  pure  white  with  which 

to  compare  the  colours.     In  the  next  experiments,  a 

strip  of  white  light  was  placed  in  juxtaposition  to  the 

colour,  and  the  results  were  slightly  different.    The  table 

below  gives  his  luminosity  measures  (Fig.  36).     Col.  I. 

is  the  empyric  scale  number,  11.  is  the  wave-length,  III. 

the  luminosity  of  the  colour  to  the  normal  eye,  IV.  the 

luminosity  to  X.,  and  V.  the  ratios  of  III.  to  IV. 

In  the  diagram,  his  luminosity  curve  X.  is  shown,  its 

area  being  1400  against  1650  for  the  normal  eye.     His 

ceiitpd^ei:ception  of  light,  as  arrived  at  by  the  extinction 


Tobacco  Blindness, 


151 


metliod,  was  only  two-thirds  of  that  of  the  normal  eye  ; 
hence  his  area  of  luminosity  should  be  1100.  As  it  is 
1400,  the  ordinates  of  the  above  curve  should  be  multi- 
plied by  0*8,  to  compare  with  that  of  the  normal  eye. 


I. 

II. 

HI. 

IV. 

V. 

Colours  to  X. 

2^ 

Luminosity 
to  the 
normal  eye. 

1 

a 

IV. 
III. 

Spectrum  colour  to 
normal  eye. 

60 

6730 

7-3 

0 

0 

Sees  only  the  white 
stripe 

Eed. 

67 

6423 

32 

10 

0-31 

Calls  red  yellowish, 
and  white  bluish 

Scarlet. 

55 

6242 

65 

38 

0-65 

»»            »> 

53 

6074 

96 

86 

0-89 

Both  one  colour 

Eed-orauge. 

51 

5920 

99 

90 

0-91 

)5                            ?> 

Orange-yellow. 

1 

6660 

92 

83 

0-90 

Calls  green  a  little 
blue ;    white    he 
sees  as  white 

Greenish-yellow. 

43 

5430 

69 

625 

0-90 

>>                           5» 

Yellowish-green. 

40 

5270 

50 

46 

0-92 

»»                          »» 

Green. 

32 

4910 

8-5 

9 

1-06 

Sees   blue  as    blue, 
and    white    yel- 
lowish 

Greenish-blue. 

31 

4960 

7 

8 

1-14 

5»                          J» 

Blue. 

26 

4680 

3 

3 

1-00 

J>                          J> 

Blue. 

His  readings  of  luminosity  were  made  without  any 
hesitation,  and  were  concordant  for  each  observation, 
which  is  not  to  be  wondered  at,  as  the  matches,  except 


152  Colour   Vision. 

at  the  blue  end,  were  practically  matches  of  different 
mixtures  of  black  and  white. 

It  appears  that  the  white  which  X.  sees  as  white  is 
the  same  as  the  orange  sodium  light,  and  that  the  red 
he  sees  is  yellowish.  The  mixture  of  this  yellowish- 
white  with  the  blue  makes  white.  He  sees  a  little 
blue  in  the  spectrum  colour  at  X  5720,  so  it  must  be 
taken  that  at  that  point  of  the  spectrum  he  begins 
to  see  colour — a  point  which  is  considerably  lower  than 
that  given  by  his  preliminary  examination  of  the 
spectrum  colour,  and  due,  no  doubt,  to  the  fact  that 
in  this  experiment  he  had  the  white  light  of  the 
positive  pole  of  the  electric  light  to  compare  with  it. 
It  seems  probable  that  w^hat  X.  called  yellowish  was 
really  a  sensation  of  white  mixed  with  a  very  small 
quantity  of  red  sensation,  for  he  saw  no  yellow  in  the 
orange,  in  which  that  colour  would  be  most  easily 
distinguished  on  account  of  its  luminosity.  Eed  light, 
when  strongly  diluted  with  white  light,  to  'the  normal 
eye  is  often  called  orange. 

As,  practically  speaking,  the  colour  vision  of  X.  is 
confined  to  blue  and  white,  it  is  of  interest  to  note  the 
difference  in  luminosity  at  the  different  parts  of  the 
spectrum  that  is  registered  by  him  and  by  P.,  who  had 
blue  (violet)  monochromatic  vision.     To  facilitate  the 


Luminosity  Curves.  153 

comparison,  the  luminosity  curve  of  the  latter  is  shown 

Fig.  37. 


in  the  diagram. 


The  thin  line  curve  is  the  normal'  curve. 


154 


Colour   Visiofi. 


Perhaps,  another  case  of  a  patient  suffering  from 
tobacco  blindness  may  be  quoted,  as  it  will  show  the 
differences  that  exist  in  recognising  the  colours  of  the 
spectrum,  and  that  the  shorter  the  visible  limit  of  the 

Table  of  Luminosity  for  G.    See  page  153. 


.9 

Colours  named  by  G. 

Colour  of  spectrum 

to  the 

normal  eye. 

57 

6423 

0 

Scarlet. 

55 

6242 

3 

No  colour 

53 

6074 

11 

Colour  "  yellow,"  white  "  blue  " 

Red-orange. 

51 

5919 

34 

»»             >>               »»           j» 

Orange-yellow. 

50 

5850 

60 

>»             »»               »»           »» 

49 

5783 

64 

Colour  "gold,"  white  "sky-blue  " 

Yellow^ 

45 

5538 

59 

40 

6270 

40 

Both  white 

Green. 

35 

5042 

18 

j» 

30 

4848 

10 

5> 

29 

4807 

6 

Colour  "  very  pale  blue,"  white 
as  white 

Blue. 

26 

4707 

4 

Culour  "  blue,"  white  "  white  " 

20 

4518 

3 

»»                 n              »>                 )» 

10 

4248 

2 

»»                 »>              j>                 >» 

Violet. 

spectrum  at  the  red  end,  the  more  pronounced  is  the 
extent  of  the  colour  blindness.  G.  suffered  from  a  very 
well-marked  tobacco  scotoma,  occupying  a  consider- 
able area.  His  curve  of  luminosity  of  the  spectrum  is 
shown  in  Fig.  37.     The  horizontal  band  beneath  will 


Progressive  Atrophy.  155 

show  the  colours  which  the  spectrum  colours  appeared 
to  match. 

G.  was  tested  for  light  sense  by  the  extinction 
method,  and  it  appears  that  the  final  sensitiveness  to 
light  at  the  central  part  of  the  eye  was  nearly  12  times 
less  than  a  person  possessing  normal  sense.  I  may 
mention  that  I  have  examined  one,  if  not  two  cases 
in  which  the  patient  was  not  only  tobacco  blind,  but 
also  congenitally  colour  blind.  Though  interesting  fot 
record,  they  need  not  be  given  in  full  here. 

With  these  specimens  of  examination  I  must  leave 
the  cases  of  tobacco  blindness.  Although  very  impor- 
tant, they  by  no  means  constitute  the  sole  cases  of 
colour  deficiency  due  to  disease.  I  will  give  as  an 
instance  a  case  of  loss  of  colour  sensation  due  to  pro- 
gressive atrophy  of  both  eyes  which  was  examined,  with 
Mr-  Nettleship's  aid.  When  tested  with  spectrum 
colours — a  patch  of  white  light  being  placed  in  juxta- 
position with  the  colour — it  was  found  that  W.  S.  was 
absolutely  blind  to  colour  from  26*75  (X  4733)  on  the 
scale  of  the  spectrum  to  the  termination  of  the  red  of 
his  spectrum,  which  was  close  to  63  on  the  scale  (\  7082). 
Above  scale  No.  26*75  W.  S.  saw  blue,  and  his  spectrum 
was  continued  normally  in  the  violet.  His  luminosity 
curve  (Fig.  37)  was  made  without  any  difficulty,  and, 


156 


Colour   Vision, 


compared  with  my  own,  shows  a  slight  deficiency  in 
brightness  from  the  red  to  the  yellow,  but  his  percep- 
tion of  luminosity  increases  as  the  blue  is  approached. 


Table  of  Luminosity  for  W.  S.     See  page  155. 


i:3 

tb 

n 

Spectrum  colours  named  by  W.  S. 

Spectrum  colours 
to  normal  eye. 

60 

6728 

3-4 

Grey 

Scarlet. 

58 

6520 

15-0 

»> 

h^ 

6330 

41-0 

»» 

55 

6242 

43 

»5 

64 

6152 

69 

52 

5996 

94 

50 

5850 

100 

>» 

Orange. 

48 

5720 

96 

45 

5538 

88 

-42 

5373 

74 

40 

5270 

61-5 

" 

Green. 

38 

5172 

45 

35 

5042 

30 

30 

4848 

12 

?? 

Blue. 

25 

4675 

6 

Bluish 

20 

4518 

4 

15 

4376 

3 

Blue 

Violet. 

10 

4248 

2-5 

?» 

He  was  subsequently  tested  with  colour  discs — Ultra- 
marine (U),  Red-royal  (R),  Emerald-green  (G),  Chrome- 
yellow  (Y),  White  (W),  and  Black  (B).      .     . 


Progressive  Atrophy.  157- 

It  was  found  that — 
165  (U)  +  48  (K)  +  147  (G)  =  1^  (W)  +  285  (B). 

The  black  reflected  3  •  4  of  white ;  hence  the  true 
equation  is — 

(i).   165  (U)  +  48  (K)  +  147  (G)  =  84  •  7  (W)  +  275  (B). 
(ii).   120  (11)4-240  (Y)  =  196  (W)  + 164  (B)  (corrected)— 
With  260  (U)  +  100  (Y)  he  sees  blue. 

250  (U)  +  110  (Y)       „       light-blue. 
242  (U)  +  118  (Y)       „       no  blue. 

This  last  in  connection  with  (ii)  shows  that  his  blue 
perception  is  neutralised  by  the  yellow,  although  the 
yellow  to  him  was  matched  with  white. 

I  have  already  shown  you  a  chart  of  the  insensitive 
area  of  the  retina  found  in  a  tobacco-blind  case,  and  it 
may  be  advisable  that  you  should  see  an  example  of 
the  curtailment  that  exists,  both  for  light  and  colour, 
in  the  field  of  vision  of  eyes  in  which  there  is 
progressive  atrophy  of  the  optic  nerves.  The  large 
black  area  shows  the  part  of  the  field  that  was  en- 
croached upon.  The  dark  spots  show  small  areas  which 
are  also  insensitive.  The  field  for  colour  shown  by 
the  inner  shaded  area  is  also  encroached  upon,  and 
practically  the  patient  was  blind  in  a  great  part  of 
his  field.  His  form  vision  was  also  very  bad,  and 
his  colour  perception  feeble.     The  three  charts  given 


158 


Colour  Vision, 


in  these  lectures  were  brought  by  Mr.  Nettleship, 
for  the  information  of  the  Colour  Vision  Committee 
oi  the  Eoyal  Society,  and  by  his  permission  they  are  re- 
produced here. 


detail,  one  in 
which  the  sen- 
sation of  colour 
is  totally  ab- 
sent in  the  left 
eye,  the  right 
eye  being  nor- 
mal ;  and  the 
other  in  which 
there  is  colour 
blindness  of  a 
very  rare  cha- 
^  racter.  The 
first  case  is 
that  of  a  lady, 
whom  we  will 
call  Miss  W. 
It  appears  from 


Colourless   Vision,  159 

the  history  of  this  lady  that  she  had  a  slight  stroke 
of  paralysis  which  affected  her  left  side,  and  that  she 
subsequently  found   her  left  eye  was   deprived  of  all 

I  sensation  of  colour.  It  is  said  by  the  specialists  who 
examined  her  retina  that  this  is  a  case  of  atrophy 
of  the  optic  nerve.  She  had  very  little  difficulty 
in  matching  the  most  brilliant  spectrum  colours  with 
the  white  patch  of  light.  Her  curve  of  luminosity 
is  given  in  Fig.  39  (see  table,  page  228).  At  19  of 
the  scale,  which  is  well  in  the  blue,  she  had  very 
little  sense  of  light,  though  her  extinction  curve 
shows  that  it  extended  to  some  distance  beyond.  The 
eye  in  which  normal  vision  existed  was,  during  the 
examination  of  the  defective  eye,  bound  up  with  a 
handkerchief,  and  when  occasionally  she  was  allowed 
to  use  both  eyes,  her  astonishment  was  great  to  see  the 
colours  which  she  had  matched  with  the  white.  The 
curve  of  luminosity  taken  with  her  right  eye  coincided 
with  my  own,  which  throughout  we  have  taken  as 
normal.  From  her  extinction  curve  we  gather  that 
there  was  a  marked  diminution  of  sensitiveness  to  light 
in  her  left  eye  compared  with  that  of  normal  vision. 
Apparently,  in  that  eye  she  only  has  -^^  of  the  normal 
sensitiveness  to  light  near  E  in  the-;  green,  but  her 
extinction  curve  takes  the  same  general  form  as  that 


i6o 


Colour   Visio7i, 


of  the  normal  eye.  The  diflference  between  the  sets  of 
ordinates  of  the  two  indicates  the  difference  in  sensi- 
tiveness for  each  part  of  the  spectrum. 


Fig. 

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Her  persistency  curve  as  calculated  occupies  the  same 
position  and  is  of  about  the  same  dimensions,  when  tl  e 


Colourless   Vision. 


i6i 


maximum  is  made    100,  as    that    of   the    normal    eye, 
as  it  is  therefore  of  red-  and  green-blind,  and  also  of 


FiCx.  40. 


The  thin  line  curve  is  the  curve  of  luminosity  for  the  normal  eye. 


M 


1 62  Colour  Vision, 

the  two  cases  of  monocliromatic  vision.  We  have  in 
Miss  W.  a  type  of  colour  blindness  which  no  present 
theory  of  colour  vision  accounts  for  without  straining ; 
and  it  would  probably  have  to  refer  it  to  the  seat  of 
sensation  rather  than  to  the  retina  alone. 

The  second  is  a  case  of  congenital  colour  blindness 
and  with  no  trace  of  disease,  brought  by  Mr.  Nettleship 
to    the    same   Committee.     He   found    that  this  lady, 
N.  W.,  mistook  blue    for  red,  and    it  was  with  some 
curiosity    that    this    case    was    examined.      Her    first 
examination  was  as  to  colour  sense  with  the  spectrum 
colours,  a  patch  of  monochromatic  light  being  placed  in 
juxtaposition  with  an  equal  patch  of  white  light.     At 
62*5  (X6890)  of  the  scale  the  light  of  the  spectrum 
disappeared.      As  the  slit  moved  along  the  spectrum, 
and   the   white  was   approximately   reduced   to    equal 
luminosity,  she  described  all  the  red  as  grey,  and  of  the  . 
same  colour   as    the  white  until   53*5   (X  6110).     At. 
this  point  she  said  the  colour  was  brownish  compared 
with  the  white,  and  this  hue  continued  to  her  till  48  , 
on  the  scale  (X  5720),  when  she  said  the  colour  was  ^ 
"  neither  brown  nor  green,  but   both."     From  48   on  > 
the  scale  she  described  the  colour  as  green,  when  it 
changed  quite  suddenly  at  31*5  (X  4905).     From  this^ 
point  and  in  the  blue  she  again  began  to  see  grey ; 


Abnormal  Colour  Vision. 


163 


1 

id 

Colours  named  by  N.  W. 

Spectrum  colours 
to  normal  vision. 

60 

6728 

3 

Both  grey 

Red. 

68 

6520 

10 

j» 

56 

6330 

30 

»♦ 

54 

6152 

52 

Colour     "  brownish,"     white 
"  grey  " 

52 

5996 

70 

?»                              5»                              )» 

50 

5850 

81 

»)                              >»                              )> 

Orange. 

48 

5720 

87 

Colour      "  brownish  -  green," 
white  "  grey  " 

46 

5596 

90 

Colour        "  green,"        white 
"  grey  " 

44 

5481 

88 

II                          >> 

42 

5373 

82 

'J                           »> 

40 

5270 

62-5 

»j                           >» 

Green. 

38 

5172 

46 

5»                                                                »> 

35 

5042 

23 

»>                                                               »» 

32 

4924 

12-5 

"                                                               '♦ 

31 

4886 

10 

Colour      "  brownish  -  grey," 
white  '*  brownish-green." 

30-5 

4862 

8-5 

?>                                     5>                                     ?> 

Blue. 

25 

4675 

5 

?»                                     1»                                     )» 

20 

4518 

3 

)»                                     J>                                     J> 

15 

4376 

2-5 

»»                                     5>                                     >» 

10 

4248 

1-5 

5>                                        J»                                        »» 

Violet. 

0 

4010 

0.| 

»>                                       »>                                       >> 

the  grey  at  this  end  of  the  spectrum,  and  also  of  the 
white  patch,  she  called  brownish-grey.  This  name 
must  evidently  have  been  a  mental  distinction,  as  she 

M  2 


164  Colour  Vision, 

described  the  red  end  and  the  white  as  grey  only,  and 
not  brownish-grey ;  and,  indeed,  she  was  tested  again 
over  that  part  of  the  spectrum,  and  adhered  to  the 
previous  naming.  It  would  appear  to  be  due  to  low 
luminosity,  which  made  the  grey  appear  to  her  what  she 
called  brownish,  rather  than  to  any  actual  difference  in 
hue. 

Her  curve  of  luminosity  in  the  spectrum  was  next 
taken,  and  her  readings  are  given  in  the  table  above. 
The  curve  is  shown  in  Fig.  40.  The  shaded  band 
beneath  it  applies  to  her  curve.  Miss  W.'s  luminosity 
curve  is  also  repeated  in  the  same  figure  for  the  sake  of 
comparison. 

An  endeavour  was  made  to  form  a  series  of  colour 
equations  with  her  eyesight  by  placing  three  slits  in 
different  parts  of  the  spectrum,  but  without  success, 
although  a  match  with  white  was  made  in  two  posi- 
tions. One  slit  was  in  the  orange-red  (52  of  the  scale), 
another  at  E,  and  the  third  at  G  ;  mixtures  were  made 
which  she  said  matched  the  white,  but  they  were  so 
erratic  that  it  was  useless  to  measure  the  apertures. 
When  the  slit  in  the  violet  was  covered  up,  a  white 
patch  being  alongside  as  a  comparison,  she  called  the 
mixture  of  red  and  green  "  brownish-green " ;  when 
-the  slit  in  the  red  was  covered  she  called  the  mixed 


Abnormal  Colour   Vision.  165 

light  of  green  and  violet  "  green "  ;  and  when  the 
green  slit  was  covered  up  she  called  the  purple  colour 
a  "  different  kind  of  brown." 

When  the  first  slit  was  moved  into  the  red  near  the 
lithium  line  she  called  the  colours  "  green,"  whenever 
the  green  slit  was  uncovered.  A  piece  of  red  glass 
was  placed  in  the  white  reflected  beam,  forming  a  red 
patch,  and  a  patch  of  the  blue  at  scale  No.  30*5 
(X  4862)  was  placed  alongside,  and  she  matched  them 
in  luminosity  and  in  colour.  (The  dominant  colour 
of  the  signal  glass  in  question  was  X  6220.)  She 
finally  was  tested  with  colour  discs. 

To  make  white  she  required 

130  G  +  113  R  +  117  U  =  72  W  +  218  B. 

She  was  then  tried  with  the  blue  and  green  discs 
alone  and  made  a  match — 

258  U  +  102  G  =  65  W  +  295  B. 

An  attempt  was  made  to  match  with  the  green  and 
red  discs  alone,  but  this  failed. 

She  matched  the  red  disc  alone  with  black  and  white, 
and  also  the  blue  disc  alone — 

360  R  =  56  W  +  304  B  (corrected), 

360  U  =  60  W  +  300  B  (corrected). 

With  any  proportion  of  R  and  U  mixed  together  she 

matched  a  grey  of  approximately  the  same  intensity  as 


1 66  Colour  Vision, 

above,  as  it  might  be  supposed  she  would  from  the 
last  two  equations. 

Taking  the  intensity  curve  of  the  light  reflected  from 
the  red  disc,  it  was  found  to  contain  a  great  deal  of  the 
part  of  the  spectrum  which  she  called  brownish,  viz., 
from  33  •  5  to  48  on  the  scale,  whereas  the  blue  reflected 
a  trifle  of  this  portion  of  the  spectrum,  as  did  also  the 
green ;  and  this  may  account  for  her  making  a  match  to 
grey  of  U  and  G,  and  not  of  R  and  Gr,  but  it  is  hard  to 
see  why  she  matched  U  alone  and  also  R  with  the  grey. 
'  Reviewing  the  case,  it  seems  that  any  perception  of 
colour  is  very  small,  and  that  the  sensations  are  green 
and  much  less  red.  From  the  equations  it  also  seems 
that  she  would  have  matched  green  with  white  and  black 
alone,  and  that  360  G  =  75  W  +  285  B.  Perhaps  the 
explanation  of  the  matches  and  names  of  colours  may 
be  that  a.  proportion  of  colour  may  be  mixed  with 
another  without  being  perceived,  but  this  colour  so 
hidden  has  still  the  capability  of  neutralising  a  certain 
quantity  of  the  complementary  colour. 


(  i67  ) 


CHAPTER    XIII. 

You  have  been  taken  through  much  experimental  work, 
and  possibly  it  may  be  thought  that  there  has  been  too 
much  of  it ;  but  now  that  we  are  coming  to  the  more 
practical  part  of  the  subject,  it  will  become  apparent 
that  a  good  working  hypothesis  is  absolutely  necessary 
before  effectual  tests  for  colour  vision  can  be  carried 
out,  and  that  the  reasons  for  its  adoption  should  be 
given  in  full.  The  question  of  colour  blindness  is  one  of 
very  practical  importance,  as  in  certain  occupations  it  is 
essential  that  colours  should  be  accurately  and  quickly 
known,  and  that  no  guess-work  should  be  allowed. 
Lives  have  without  doubt  been  lost  by  a  want  of  proper 
knowledge  of  colours,  both  at  sea  and  on  railways. 
The  evidence  that  such  is  the  case  is,  as  a  rule,  it  is 
true,  merely  negative,  though  there  are  cases  extant 
where  great  losses  which  have  occurred  can  be  traced 
to  a  deficiency  in  colour  perception.  If  there  be  no 
proper  system  of  tests  for  ascertaining  the  defects  of 


1 68  Colour   Vision, 

signal  or  look-out  men  in  their  colour  sense,  it  is 
palpable  that  positive  evidence  cannot  be  forthcoming, 
and  this  is  very  much  the  state  of  things  which 
exists  up  to  the  present  time.  We  hear  of  collisions 
at  sea  and  vessels  foundering  in  consequence  of  the 
rule  of  the  road  not  being  followed,  but  at  the  in- 
vestigations which  follow  we  have  no  record  that  the 
question  of  colour  perception  of  the  look-out  man  has 
been  gone  into,  though  there  may  be  conflicting  evi- 
dence as  to  whether  a  red  light  or  a  green  light  was 
shown.  That  danger  from  colour  blindness  is  incurred 
has  for  some  time  been  recognised  by  the  Board  of 
Trade,  as  it  insists  that  all  officers  of  the  Mercantile 
Marine  must  be  tested  for  their  sense  of  colour,  and 
that  their  certificates  must  be  endorsed  as  having  failed 
to  pass  the  colour  test  should  they  do  so.  For  my  own 
part,  I  think  endorsement  of  their  certificate  is  quite 
inadequate,  for  it  is  still  open  for  shipowners  to  employ 
them  (of  course  at  their  own  risk).  A  rejection  for 
colour  vision  should  entail  a  withholding  of  the  certifi- 
cate altogether ;  for  it  surely  is  as  dangerous  that  a 
signal  should  be  misread  as  it  is  that  the  logarithm  of 
the  sine  of  an  angle  should  be  misunderstood.  If  a 
candidate  fails  in  theoretical  navigation,  he  is  not 
allowed   a   certificate,  but   if  he  only  fails  in  a  very 


Holmgren  Tests.  169 

practical    part    of    his   examination,   his    certificate    is 
merely  endorsed. 

The  system  employed  by  this  department  was  a 
defective  one,  and  we  know  of  many  instances  in  which 
candidates  have  passed  the  colour  test,  though  they 
ought  to  have  been  rejected,  and  are  at  present  in  the 
service.  The  subject  of  testing  for  colour  vision  was 
brought  prominently  forward  some  two  or  three  years 
ago,  and  a  Committee  of  the  Eoyal  Society,  to  which 
I  acted  as  secretary,  was  requested  to  consider  the 
methods  at  that  time  in  force  on  the  railways  and  in 
the  mercantile  marine,  and  to  find  one  which  was  not 
open  to  objection.  It  recommended  the  system  that 
had  been  elaborated  by  Holmgren,  a  Swedish  physicist, 
and  known  as  Holmgren's  test,  which  has  long  been 
in  force  in  Sweden  and  elsewhere.  This  system  has, 
I  am  glad  to  say,  been  adopted  by  the  Board  of 
Trade,  and  by  most  of  the  railway  companies  in  the 
United  Kingdom.  There  have  been  numerous  indica- 
tions that  this  change  of  method  was  necessary.  Only 
within  the  last  month  (April,  1894),  for  instance,  I  was 
informed  by  the  Medical  Officer  who  had  to  examine 
the  employes  on  a  certain  railway  in  Scotland  by  the 
Holmgren  test  that  he  had  found  some,  amongst  others 
an    engine-driver,    who    were   colour   blind,    and   pre- 


170  Colour   Vision, 

sumably  unfit  for  tlie  posts  they  occupied  owing  to  this 
defect. 

There  is  one  popular  objection  which  is  always  made 
against  this  test,  or  indeed  against  any  proper  test, 
viz.,  that  the  examination  is  not  made  under  the 
same  conditions  which  absolutely  exist,  nor  with  the 
very  lights  which  the  candidates  have  to  distinguish 
from  one  another — that  is,  the  red  and  green  lights. 
Let  me  beg  of  you  to  remark,  that  as  a  mere  matter 
of  guessing,  the  chances  are  equal  thafc  a  man  would 
name  the  light  shown  correctly.  If  you  turn  a  man's 
back  to  the  light,  and  if  he  has  a  coin  in  his  pocket  and 
deliberately  calls  heads  red  and  tails  green,  he  will  have 
a  good  chance  of  passing  the  test ;  for,  if  he  guessed 
rightly  three  or  four  times,  no  one  would  fail  to  pass 
him  on  his  answers.  The  great  point  in  a  test  is  to 
cause  the  candidate  to  do  something  to  show  that  he 
appreciates  colour.  It  is  this  doing  something  and 
saying  nothing  which  is  the  important  feature  in  the 
Holmgren  test.  A  man  may  be  ignorant  of  the  names 
of  colours — colour  ignorant  it  is  called — but  he  cannot 
be  ignorant  of  the  colours  themselves  if  he  has  normal 
colour  vision.  As  a  matter  of  fact,  the  colour  blind 
may  possibly  distinguish  between  red  and  green  lights 
by  having  carefully  noted,  under   ordinary  conditions 


Basis  for  Tests.  171 

of  atmosphere,  their  different  brightness,  and  by  their 
rlifference  in  saturation  with  their  neutral  colour.  If 
external  conditions  are  altered,  as  they  are  in  actual 
daily  life,  these  slight  indications  vanish,  and  the  quick 
naming  of  the  colour  to  be  read  becomes  a  mere  matter 
of  chance.  A  proper  test  should  include  all  variations 
that  can  occur  in  these  respects.  It  cannot  be  too 
strongly  impressed  upon  every  one  that  a  man  who  is 
colour  blind  to  colours  in  ordinary  daylight  is  equally 
so  in  lamplight,  although  some  shades  of  colour  which 
are  well  distinguishable  by  daylight  may  disappear 
when  the  artificial  light  is  used  as  the  source  of 
illumination. 

Now,  on  what  scientific  principles  should  a  colour 
test  be  founded?  We  must  hark  back  to  a  theory 
for  a  moment,  and  as  it  has  been  shown  that  for  all 
essential  purposes  that  of  Young  answers,  we  will  use 
it  as  a  good  working  hypothesis,  and  it  was  from  this 
theory  that  Holmgren  himself  reasoned.  The  red-  or 
green-blind  see  a  grey  in  a  part  of  their  spectrum, 
which  to  us  who  possess  normal  colour  vision  is  green. 
If  then  we  present  such  a  green  to  them,  they  would 
match  it  with  a  grey.  If,  however,  we  have  a 
yellowish-green,  which  is  pure  green  mixed  with  red,  the 
complete  green-blind  will  not  see  the  green  in  it,  but 


172  Colour  Vision. 

only  the  red.  The  colour  to  him  would  be  very  pale  red, 
and  as  he  sees  all  such  greens  and  yellows  and  reds  as 
red  more  or  less  saturated,  that  is,  more  or  less  mixed 
with  his  neutral  colour,  any  one  of  these  he  would  match 
with  a  green.  The  red-blind,  on  the  other  hand,  would 
see  all  these  colours  as  green,  and  he  too  might  make 
similar  matches  with  them.  Suppose  now  we  have  a 
pink  skein  :  the  green-blind  would  see  it  as  a  white  or 
bluish- white,  for  a  purple  is  white  to  him,  and  he  would 
match  with  it  either  greys  or  colours  having  a  slight 
excess  of  blue  in  them  ;  for  a  green  is  to  him  a  neutral 
colour.  The  red-blind,  on  the  other  hand,  would  see 
but  little  green  in  the  pink ;  blue  would  predominate, 
so  he  would  choose  mauves  or  blues  amongst  other 
matches. 

Acting  on  these  principles,  Holmgren  selected  his 
test  colours.  He  chose  wools  as  the  most  convenient 
for  handling,  and  also  because  they  present  the  same 
colour  without  sheen  when  looked  at  in  any  direction. 
His  first  test  colour  is  a  very  pale  green  which  con- 
tained no  blue.  Its  paleness  is  a  point  in  its  favour. 
The  colour  is  quite  distinguishable  by  us  normal-visioned 
persons,  but  it  might  appear  as  grey  to  the  red-  and 
green-blind ;  for  as  we  who  possess  normal  vision  may 
mix  a  small  percentage  of  colour  with  our  neutral  colour 


Test  Skeins,  173 

(white)  without  it  being  perceived,  so  may  they  with 
theirs  (white  and  green).  As  the  green,  when  it  is  to  us 
saturated,  would  be  nearly  neutral  coloured  to  them,  the 
very  diluted  colour  which  we  see  in  the  skein  would  to 
them  be  masked  by  the  addition  of  white.  In  any  case, 
if  any  colour  be  visible  to  them,  it  must  be  on  the  red 
side  of  the  neutral  points.  A  candidate  is  given  this 
skein  of  wool,  and  from  a  heap  of  over  a  hundred  skeins, 
of  varying  degrees  of  saturation,  amongst  which  are 
drabs,  yellows,  yellow-greens,  blue-greens,  purples,  pinks, 
greys,  and  so  on,  he  is  asked  to  select  others  which 
appear  to  him  to  be  of  the  same  colour  as  the 
test-skein,  though  they  may  be  darker  or  lighter. 
He  will,  if  colour  blind,  select  some  of  the  colours 
already  indicated.  The  second  test-skein  is  a  pink, 
which  is  a  purple  diluted  with  white,  but  much 
less  so  than  the  green,  to  which  it  is  nearly  a  com- 
plementary in  daylight.  The  candidate  is  required  to 
select  colours  which  match  this,  and  according  to  his 
selections  is  he  pronounced  as  having  normal  colour 
vision  or  as  being  colour  defective  (either  completely  or 
partially)  to  the  red  or  to  the  green.  The  case  of  violet 
blindness  is  not  important  in  reading  the  signals  or- 
dinarily used,  and  therefore  in  this  test  no  special  test- 
skein  is  employed.     Let  us  consider  what  colour  we 


174  Colour  Vision. 

should  use.  The  neutral  colour  to  this  form  of  colour 
blindness  is  yellow.  If,  therefore,  we  pick  out  a  pale 
yellow  skein,  the  candidate  would  pick  out  greys  to 
match  it;  or  if  we  gave  him  the  pink  skein  to 
match,  since  he  has  no  blue  (violet)  sensation,  he 
would  match  it  with  a  pure  red  or  with  a  purple. 

Where  monochromatic  vision  is  under  examination, 
all  skeins  would  be  matched  with  one  another  indis- 
criminately— blues,  reds,  greens,  greys  will  all  be  a 
match,  some  lighter  and  some  darker  than  the  test- 
skein.  I  have  been  told  by  some  who  have  carried 
out  examinations  for  colour  blindness  that  this  matching 
is  by  no  means  so  uncommon  as  is  often  imagined.  In 
future  it  is  hoped  that  most  of  those  who  make  these 
matches  may  be  examined  by  the  spectrum  test,  as  it 
may  turn  out  that  a  proportion  of  them  will  be  most 
valuable  theoretical  cases. 

In  making  an  examination  with  the  Holmgren 
test,  it  is  almost  unnecessary  that  the  candidate  should 
take  up  a  skein  out  of  the  heap  of  wools  to  form 
a  preliminary  diagnosis.  The  colour  blind  will  not 
at  once  pick  out  an  evident  match,  but  will  hesitate 
and  evince  a  desire  to  appear  very  accurate  in  his 
choice.  This  indicates  at  once  that  there  is  something 
amiss.     He  probably  will  pick  up  a  skein  of  the  right 


Practical  Testing.  175 

colour,  place  it  against  the  test-skein,  lay  it  down 
and  again  take  it  up.  Or  he  will  pick  up  a  skein 
which  is  evidently  incorrect  and  do  the  same  thing, 
but  perhaps  he  will  return  it  to  the  heap  and  take 
up  another  which  is  equally  bad. 

He  will  fumble  over  making  his  matches,  and 
eventually  have  a  heap  by  him  which  will  at  once 
tell  the  examiner  that  he  is  colour  defective.  I  may 
as  well  give  you  an  idea  of  the  colours  which  the 
colour  blind  will  pick  out  by  a  simple  experiment. 
The  heap  of  wools  is  on  the  table,  and  in  the  pure 
white  of  the  electric  arc  light,  which  is  thrown  on  it 
from  the  lantern,  every  colour  is  distinct  in  hue  and  in 
intensity.  On  one  side  are  placed  the  two  important 
test-skeins,  the  pale  green  and  the  pink.  There  can 
be  no  doubt  but  that  in  that  heap  of  wools  there  are  a 
large  number  which  can  be  matched  with  each  of  them. 
The  red-blind,  be  it  recollected,  sees  no  red,  and  if  I 
can  place  in  front  of  the  lens  of  the  lantern  some 
medium  which  cuts  off  the  red  as  completely  as 
possible,  the  audience  as  well  as  myself  will  see  the 
colours  approximately  as  the  red-blind  would  do.  Such 
a  medium  is  found  in  the  same  blue-green  glass  that  is 
used  for  signals  on  most  railways  and  on  board  ship. 
The  green-blind,  on  the  other  hand,  see  no  green,  and  if 


176  Colour  Vision, 

a  medium  can  be  found  which  when  placed  in  the  path 
of  the  light  allows  no  green  to  pass,  the  colours  in  the 
heap  being  deprived  of  the  green  would  be  such  as 
would  very  nearly  be  the  same  as  this  type  of  colour 
blind  would  see.  This  glass  is  covered  with  a  film  of 
collodion  in  which  fuschin  and  blue  have  been  dissolved. 
It  transmits  a  fine  purple  and  should  answer  our  pur- 
pose. That  these  two  media  are  what  we  require 
can  be  readily  demonstrated  by  placing  them  in  front 
of  the  slit  of  the  collimator  of  our  colour  apparatus  and 
throwing  the  spectrum  on  the  screen.  The  spectrum 
of  white  light  is  now  on  the  screen,  and  when  we  place 
the  blue-green  glass  in  front  of  the  slit,  we  see  that  the 
red  is  very  nearly  entirely  extinguished,  w^hilst  if  we 
substitute  for  it  the  dyed  collodionized  glass  the  green 
is  absent.  Now,  placing  the  first  glass  in  front  of  the 
lantern  lens  and  switching  on  the  current,  the  wools  are 
illuminated  with  the  bluish-green  light.  The  green  test- 
skein  appears  green,  and  we  can  proceed  to  make  our 
matches,  picking  out  the  colours  which  appear  the  same, 
but  taking  no  heed  as  to  their  lightness  or  darkness. 
A  dozen  skeins  are  now  picked  out,  and  I  think  the 
audience  will  agree  with  me  that  the  matches  as  viewed 
in  the  green  light  are  accurate.  The  glass  is  now  with- 
drawn, and  the  ordinary  white  light  falls  upon  the  skeins 


Matching  Colours.  177 

m  my  hand.  They  are  a  strangely  variegated  lot  as  now 
seen  ;  we  have  green  shades,  yellows,  and  browns,  and 
greys.  Such  a  variety  would  tell  me  that  I  was  colour 
deficient,  but  would  not  be,  perhaps,  decisive  as  to  what 
was  the  exact  character  of  the  deficiency.  For  if  the 
pink  glass  is  placed  in  front  of  the  lantern  you  will  find 
the  same  matches,  with  one  or  two  exceptions,  might 
have  been  made.  The  blue-green  glass  is  once  more 
placed  in  the  beam,  and  this  time  I  match  the  pink  skein 
with  the  wools.  A  certain  number  are  picked  out,  and 
the  audience  will  agree  with  me  that  the  matches  are  fair 
ones.  When,  however,  the  glass  is  withdrawn  from  the 
light  and  we  see  what  colours  have  been  selected,  we 
find  that  they  consist  of  pale  blues,  mauves,  pinks  of 
various  shades,  and  cerise,  and  violet.  The  red  in  the 
pink  did  not  afiect  my  eyes  any  more  than  would  it 
the  red-blind.  I  am  evidently  then  in  this  light  red- 
blind,  for  if  the  pink  glass  replaces  the  blue-green,  the 
matches  are  impossible.  While  this  coloured  light  is 
illuminating  the  heap  I  will  make  matches  again. 
When  made,  the  w^hite  light  is  again  thrown  on  the 
selected  skeins,  and  this  time  we  have  bluish-green 
and  neutral  tint  together  with  pinks.  The  reason  of 
this  is  evident,  there  is  no  green  visible  ;  the  bluish - 
green  contains  besides  blue  a  certain  amount  of  yellow, 

N 


1 78  Colour   Vision. 

which,  in  its  turn,  contains  red,  and  the  grey  must  be 
pink.  To  the  green-blind,  for  reasons  already  given, 
the  blue-green  looks  white,  as  does  the  pink,  and  there- 
fore the  two  are  matched  together.  The  grey  is  also 
degraded  white  to  him,  and  therefore  he  also  matches 
that  with  them.  The  matches  which  the  violet-blind 
would  make  can  be  well  exemplified  by  placing  in  the 
beam  of  light  a  yellow  glass,  or  a  glass  coated  with 
collodion  in  which  "  brilliant  yellow "  has  been  dis- 
solved. By  this  plan,  then,  we  can  in  some  measure 
produce  the  effect  of  colour  blindness  on  ourselves, 
and  very  interesting  it  is  to  compare  theory  with 
the  results  obtained  in  this  manner.  There  is  no  ne- 
cessity to  have  recourse  to  the  electric  light  for  this 
purpose.  If  matches  are  made  with  such  media  held 
before  the  eyes  in  ordinary  daylight,  the  same  results 
will  be  obtained.  I  have  often  examined  through  these 
same  media  the  matches  made  by  the  colour  blind,  and 
been  able  at  once  to  settle  the  nature  of  the 
defective  vision  from  which  they  were  suffering. 
It  must  be  remembered  that  the  colours  transmitted 
through  these  two  glasses  are  not  absolutely  like  the 
whites  which  the  two  classes  of  colour  blind  see  respec- 
tively, though  they  approach  it. 

We  can  imitate  even  more  exactly  the  matches  that 


Matching  Colours.  179 

they  would  make  by  matcliing  white  light  with  a 
mixture  of  red,  green,  and  violet  of  the  proper  hues, 
and  covering  up  the  red  or  green  slit,  and  then  placing 
the  test-skein  and  the  matches  in  the  colour  so  formed. 
From  the  other  skeins  viewed  in  the  same  light  can 
be  picked  out  the  matches  which  would  be  possible. 
There  is  very  little  chance,  if  any,  of  a  mistake  about 
them  being  made  when  this  plan  is  adopted. 


y  2 


(   i8o  ) 


CHAPTER    XIV. 

Holmgren's  test,  although  a  qualitative  one,  is  most 
accurate  in  allowing  a  diagnosis  to  be  formed,  but  it 
sometimes  happens  that  a  candidate  is  not  satisfied  that 
he  has  failed  in  passing  the  test,  and  wishes  for  another 
examination.  Such  a  re-examination  is  best  carried  out 
by  the  spectrum  method,  which  I  will  now  describe. 

The  test  with  the  spectrum  is  a  very  decisive  one, 
and  can  be  carried  out  with  the  patch  apparatus  (Fig.  3), 
page  19.  Personally,  I  like  to  have  some  idea  of  the 
kind  of  colour  blindness,  if  any,  which  exists  by  first 
using  the  Holmgren  test.  Should  these  tests  show  that  a 
candidate  is  colour  blind  in  any  degree,  a  very  excellent 
beginning  is  to  try  and  find  his  neutral  point  in  the 
spectrum — if  he  has  one.  To  arrive  at  it  we  place  two 
patches  of  light  on  the  screen,  one  of  colour  and  the 
other  of  white,  the  rotating  sectors  being  in  the  last- 
named  beam,  and  ask  him  to  say  when  the  two  colours 
appear  alike.  It  must  be  remembered  that  white  is 
coloured   from  the  efiect  of   contrast  as   long   as    the 


spectrum   Test.  i8i 

colour  alongside  differs  from  it.  A  good  point  de 
depart  is  with  the  slit  in  the  yellow,  then  to  move  it 
into  the  red,  and  then  gradually  to  push  it  into  the 
green.  When  here,  if  colour  blind,  he  will  say,  '^  The 
two  patches  are  nearly  alike,  but  that  the  white  is 
rather  pink  or  green,"  as  the  slit  gets  further  towards 
the  blue.  The  operator,  whilst  changing  the  colour, 
alters  the  sectors  so  that  the  luminosities  are  about  the 
same.  A  point  will  be  reached  when  the  colour  blind 
will  say,  ''  Now  they  are  both  alike,  but  one  is  rather 
darker  than  the  other."  The  sectors  are  altered  until 
he  says  they  are  both  alike,  and  the  observation  is 
satisfactory  when  he  declares  the  two  patches  of  light 
are  both  alike  in  colour  and  in  darkness.  It  is  curious 
how  misleading  the  word  brightness  is  to  some  people 
who  are  uneducated.  I  find  it  much  safer  to  ask 
which  is  the  darker  colour,  rather  than  which  is  the 
brighter.  A  little  patience  will  always  enable  you  to 
get  a  good  observation.  The  place  in  the  spectrum 
which  is  the  neutral  point  is  now  noted.  The  neutral 
point  is  again  found,  but  this  time  commencing  in 
the  blue.  The  same  procedure  is  adopted  as  before, 
and  we  thus  get  a  second  reading  for  it,  and  the 
two  will  be  found  to  be  very  close  to  one  another. 
In   difficult   cases,  four   or   five   observations   may   be 


1 82  Colour  Vision. 

made,  and  the  mean  taken  as  a  close  approximation. 
So  far  the  spectrum  test  has  not  shown  whether  the 
observer  is  red-  or  green-blind,  except  by  comparing 
the  position  of  the  neutral  point  with  that  usually 
found  by  the  two  types.  We  have,  however,  an  un- 
erring criterion  by  the  luminosity  method.  The  red 
is  placed  beside  the  white,  and  he  is  asked  to  say 
which  he  considers  the  darker ;  he  will  give  an  answer 
of  some  kind,  and  probably  protest  that  the  two 
colours  are  not  alike.  A  soothing  answer  will  disarm 
his  objection,  and  he  will  quickly  see  what  you  mean. 
If  he  be  red-blind  he  will  match  in  brightness  a  bril- 
liant red  and  a  feeble  white  ;  if  he  be  green-blind  he 
will  make  a  match  very  similar  to  normal  vision.  In 
the  case  of  the  red-blind  the  slit  is  then  moved  into 
the  extreme  red,  when  he  will  say  he  sees  but  one 
patch  of  light,  whilst  the  green-blind  will  see  it  as  a 
person  of  normal  vision  would  do.  If  time  permits,  the 
whole  luminosity  curve  may  be  taken  and  registered. 
This  is  not  essential,  but  interesting  for  reference. 
Where  complete  colour  blindness  exists,  it  should  be 
possible  to  cause  him  to  match  a  green  with  a  red. 
To  do  this  a  second  instrument,  as  described  in  page 
18,  may  be  used,  but  it  is  quite  sufficient  if  a  piece 
of  red  glass,  such  as  is  used  for  railway  signals,  or  of 


Matching  Green  with  Red.  183 

bottle-green  glass,  be  placed  in  the  white  beam.  There 
is  then  a  red  or  green  patch  alongside  the  patch  of 
spectrum  colour.  The  red  will  stimulate  the  red  sen- 
sation of  the  green-blind,  but  not  being  spectrum  red  it 
contains  a  certain  amount  of  yellow,  which  stimulates 
the  green  sensation  if  the  observer  be  red-blind.  The 
green  is  of  such  a  colour  that  it  will  stimulate  both 
the  red  and  the  green  sensations.  In  the  path  of  the 
reflected  beam  between  G  and  the  prisms  (Fig.  3)  a 
sheet  of  plain  glass  is  inserted,  which  reflects  a  propor- 
tion of  white  on  to  the  red  patch.  The  sectors  are  placed 
in  this  beam.  If  the  red  glass  is  being  used,  the  slit  is 
moved  into  the  green  near  E,  and  the  colour  blind  will 
say  that  both  are  the  same  colour,  but  one  darker  than 
the  other.  By  opening  or  closing  the  slit  in  the  spec- 
trum, he  will  possibly  say  that  both  colours  are  alike 
and  of  the  same  darkness,  but  he  may  say  one  is  paler 
than  the  other,  in  which  case  the  white  light  must  be 
increased  or  diminished  by  means  of  the  sectors  till 
equality  of  tone  is  established.  This  applies  to  the 
red-blind  and  the  green-blind.  The  former  will  require 
a  very  bright  red  to  match  a  feeble  green,  whilst  with 
the  latter  the  red  will  require  a  fairly  light  green. 
When  the  green  glass  is  used  the  spectrum  colour  patch 
should  be  red,  and  the  match  be  made  as  before.     With 


184  Colou7'   Vision. 

the  violet-blind  the  neutral  point  will  be  in  the  yellow, 
and  with  monochromatic  vision  matches  can  be  made 
throughout  the  spectrum.  So  far  it  will  be  seen  that 
no  mention  of  any  colour  is  required.  It  may  next 
be  advisable  to  ask  him  the  names  of  colours.  This  is 
best  done  by  placing  the  white  patch  of  light  over  the 
spectrum  colour  patch,  and  opening  and  closing,  as 
may  be  required,  the  sectors.  If  the  sectors  are 
closed  it  is  very  probable  that  correct  guesses  may  be 
made,  for  then  the  colours  will  be  saturated,  and  the 
colour  blind,  if  they  are  intelligent,  will  know  that  a 
green  to  them  is  white  or  pale  in  colour  compared 
with  red,  though  of  the  same  hue.  If  white  be  mixed 
with  the  red  the  wrong  name  is  bound  to  be  given,  for 
they  will  be  unable  to  distinguish  it  from  the  green, 
because  it  is  then  a  less  saturated  colour.  Passing  from 
green  to  red  and  mixing  the  colour  more  or  less  with 
white,  the  most — I  was  going  to  say  grotesque — telling 
mistakes  are  made.  A  further  excellent  test  is  to  place 
a  cell  containing  a  solution  of  bichromate  in  the  path 
of  the  reflected  beam,  and  cause  the  observer  to  match 
its  colour  with  the  light  coming  through  two  slits,  one 
in  the  red  near  C,  and  the  other  in  the  green  near  E. 
Defective  colour  perception  will  be  well  demonstrated. 
There  are  various  other  artifices  which  can  be  employed 


Malmgerers.  185 

in  the  spectrum  test,  which  would  be  too  long  to  re- 
count here,  and  if  there  be  two  sets  of  apparatus  the 
tests  are  practically  unlimited  in  number. 

There  are  cases  in  which  an  observer  who  may 
have  normal  vision  may  wish  to  be  reported  as  colour 
blind.  A  seaman's  life  is  not  always  a  happy  one, 
and  a  boy  on  a  training-ship,  knowing  that  a  failure  in 
colour  vision  will  free  him  from  a  sea  life,  may  be 
anxious  to  be  told  he  has  failed  in  colour  vision.  By 
"  coaching "  in  the  Holmgren  test  he  might  manage 
to  obtain  a  "  failure,"  but  a  malingerer  is  sure  to  be 
detected  by  the  spectrum  method  of  testing.  He  may 
call  diluted  red  green,  and  he  may  declare  he  sees 
a  neutral  point  in  the  spectrum,  but  if  he  be  tested 
with  the  diluted  colours  near  his  supposed  neutral 
point,  he  is  sure  to  fall  into  a  trap.  He  will  make  a 
mistake  in  calling  a  patch  green  when  it  ought  to 
be  white,  or  white  when  it  ought  to  be  green,  if  he 
were  truly  colour  deficient — indeed,  a  malingerer  has  no 
chance  of  escaping  detection  with  the  spectrum  tests. 
It  is  not  an  uninteresting  experiment  to  get  an  acute 
observer  who  has  normal  colour  vision,  and  is  accus- 
tomed to  the  spectrum  test,  to  feign  colour  blindness, 
and  examine  him  in  this  manner.  He  never  fails  to 
make  such  mistakes  as  would  lead  to  his  detection. 


1 86  Colour   Vision. 

With  the  partially  colour  blind  the  same  procedure 
may  be  adopted.  In  examination  by  the  Holmgren 
wool  test,  slight  mistakes  will  be  made  in  matching  the 
first  two  test-skeins.  With  the  spectrum  test  the  red 
will  require  a  greater  dilution  with  white  before  it  will 
be  matched  with  a  green,  even  if  it  can  be  matched  at 
all.  Measures  of  the  luminosity  at  four  or  five  posi- 
tions in  the  spectrum,  extending  from  near  the  extreme 
red  to  the  blue,  will  give  an  unerring  criterion  of  the 
kind  and  extent  of  colour  blindness  from  which  they 
are  sufiering.  The  existence  of  a  neutral  point  in  the 
spectrum  is  sufiicient  to  indicate  that  their  blindness 
is  of  a  nature  to  be  dangerous  in  certain  occupations. 
To  some  it  may  be  a  difiiculty  how  a  neutral  point  can 
be  found  in  such  cases,  since  all  sensations  are  more  or 
less  present.  The  reason,  however,  was  explained  on 
page  96. 


(   iS7  ) 


CHAPTER    XV. 

Examples  of  colour  blindness  have  been  brought  to 
your  notice,  and  various  measurements  made  by  per- 
sons possessing  normal  and  defective  colour  vision  have 
been  recorded,  but  no  attempt  has  been  made  to 
discuss  the  two  leading  rival  theories  that  have  been 
laid  before  you.  Regarding  these  theories  you  may 
expect  me  to  say  something,  and  to  avow  myself  a 
partisan  of  one  or  the  other.  This  last  I  must  decline 
to  do,  though  it  will  have  been  seen  by  the  line  that 
I  have  taken  in  these  lectures  that  the  Young  theory 
attracts  me.  There  are,  however,  difficulties  in  adapting 
it  to  explain  several  facts  of  colour  vision  which  seem  to 
render  it,  to  say  the  least,  incomplete.  For  instance,  to 
explain  the  colours  produced  by  simultaneous  contrast, 
the  Young  theory  has  to  betake  itself  into  psychological 
ground.  I  will  show  you  some  excellent  examples  of 
contrast  colours.  We  have  upon  the  screen  a  patch  of 
white   reflected  light,  superposed   over  a  patch  of  red 


1 88  Colour   Vision. 

light.  Placing  a  thin  rod  in  the  paths  of  the  two 
beams,  we  have  two  shadows — one  illuminated  by  white 
and  the  other  by  red,  and  lying  between  them  a  mixed 
light  of  red  and  wiiite.  The  shadow  illuminated  by 
the  white  does  not  appear  white,  but  a  bluish-grey. 
When  the  spectrum  colour  is  changed  to  orange  the 
blue  is  intensified,  whilst  when  it  is  green,  what  should 
be  white  appears  of  an  orange-salmon  colour.  Other 
colours  give  the  white  different  hues  which  I  need  not 
describe. 

These  contrast  colours  are  usually  said  to  be  comple- 
mentary to  the  spectrum  colours  employed,  though  it 
must  be  recollected  that  what  a  complementary  colour 
should  be  is  determined  by  the  quality  of  the  white 
light  which  the  two,  when  mixed,  are  made  to  match. 
But  recent  measures  of  my  own  show  that  they  are 
not  truly  complementary  in  most  instances,  whatever 
the  white  light  may  be.  But  w^hether  they  are  or  not 
does  not  much  matter  when  the  explanation  offered  by 
the  followers  of  the  Young  theory  is  considered,  for  it  is 
asserted  that  such  contrast  colours  have  no  real  existence, 
but  are  psychological,  or — -what  this  comes  to  be — simply 
delusions.  If  they  are  not  real  colours  felt  by  the 
retina,  they  have  a  very  good  resemblance  to  them,  and 
the  same  series  of  delusions  are  so  persistent  and  so 


Contrast  Colours.  1S9 

constant  for  all  normal  vision  that  they  can  always  be 
measuxed  as  having  a  constant  value.  I  bear  in  mind 
the  experiment  in  which  the  contrast  colour,  after  being 
produced,  is  isolated  in  the  eye  from  the  colour  pro- 
ducing it  and  the  background,  and  the  continuance  of 
the  hue  produced  by  the  contrast.  This  retention  may 
be  psychological,  but  there  are  no  grounds  to  my  mind 
for  saying  that  its  production  is  due  to  the  same  cause, 
more  especially  as  experiments  have  been  arranged  to 
show  that  one  eye  may  see  a  contrast  colour,  whilst  the 
other  may  see  it  of  its  uncontrasted  hue.  In  this  last 
experiment  it  can  scarcely  be  conceived  that  one  eye 
should  be  subject  to  delusion,  whilst  the  other  was  free 
from  it.  If,  then,  we  may  presume  that  they  are  real 
colours,  the  Young  theory  fails  to  explain  them,  and 
the  explanation  offered  by  the  Hering  theory  is  much 
more  acceptable,  as  it  propounds  the  idea  that  the 
retina  has  to  be  considered  as  a  whole,  and  that  if 
(say)  red  light  is  at  work  at  one  part  its  complemen- 
tary colour  (blue-green)  must  be  felt  at  another.  It 
would  be  still  more  acceptable  had  it  happened  that  the 
contrast  colours  w^ere  truly  complementary,  and  if  the 
same  action  was  noticeable  when  the  adjacent  part  of 
the  retina  was  not  also  stimulated. 

For  what    I    may  call    the   straightforward   part  of 


19^  Colour  Vision, 

colour  vision,  dealing  with  ordinarily  bright  colours,  the 
Young  theory  is  amply  sufficient ;  but  when  we  come 
to  the  feeble  luminosities  and  the  colour  fields,  it  is 
again  difficult  to  adapt  to  explain  the  phenomena  ob- 
served. When  we  reduce  the  luminosity  of  a  coloured 
ray  sufficiently  we  feel  the  sensation  of  grey  light :  no 
colour  is  felt.  Why  is  this  ?  On  the  Hering  theory  it 
is  capable  of  the  explanation  that  we  have  the  white 
sensation  left  unextinguished,  but  I  fail  to  see  any 
explanation  on  the  Young  theory.  When  we  take 
colour  fields  with  pure  colours  (see  appendix,  page 
208),  we  are  met  with  the  unexplained  difficulty  that 
the  colour  from  a  bright  spot  of  light  vanishes  almost 
suddenly  towards  the  periphery  of  the  retina,  and  is 
replaced  by  a  bright  white  light,  and  that  the  extent 
of  the  field  depends  on  the  brightness  of  the  colour. 
This,  perhaps,  is  the  most  telling  observation  which 
can  be  recorded  against  the  Young  theory  as  it  stands 
at  present.  It  has  this  support,  however,  in  the 
sequence  of  the  phenomena  observed,  viz.,  when  the 
boundary  for  the  colour  which  we  will  suppose  to  be 
pure  red  is  being  taken  (as  described  at  page  11), 
that  close  to  the  point  where  it  bursts  into  pure  white, 
it  assumes  a  pink  colour  {i.e.,  a  mixture  of  red  and 
white),  whilst,  if  the  red  be  scarlet,  containing  accord- 


Voting  V.  Hering.  191 

ing  to  this  theory  a  little  green  sensation,  it  becomes 
orange  before  white,  showing  that  the  red  sensation  is 
dimmed  slightly  before  the  green,  and  so  with  the 
other  colours.  What  are  called  ^'  after  images  "  I  have 
not  touched  upon  so  far,  nor  shall  I  here,  for  it  is  at 
this  point  that  we  step  into  very  debateable  ground. 
The  colours  perceived  in  them  are,  as  yet,  not  capable 
of  being  put  to  the  test  of  physical  measurement,  and 
I  must  leave  the  psychologist  or  the  physiologist  to 
account  for  them  in  their  own  way. 

Viewing  the  Hering  theory  from  a  physical  stand- 
point, and  in  the  light  of  colour  measurement,  it 
appears  to  be  deficient  in  several  respects.  To  take 
one  point.  We  have  seen  that  when  blue  and  yellow 
are  mixed  together  to  make  white  the  sum  of  the 
luminosities  of  the  two  colours  separately  is  equal  to 
the  luminosity  of  the  white  produced.  According  to 
the  Hering  theory,  the  yellow  colour  contains  a  cer- 
tain amount  of  the  white-black  sensation  besides  the 
yellow  sensation,  as  does  also  the  blue  colour  besides 
the  blue  sensation.  The  theory  tells  us  that  when 
white  is  produced  by  the  mixture,  the  blue  sensation 
undoes  the  work  that  the  yellow  sensation  has  done, 
and  the  white  sensation  is  alone  left  behind.  If  this 
be  the  case,  the  sum  of  the  separate  luminosities  can- 


iQ^*  Colour   Vision: 

not  be  the  same  as  tliat  of  the  white  produced,  but 
should  be  greater.  The  theory  also  has  to  be  strained 
sometimes  to  make  it  fit  in  with  other  observed  facts. 
Take,  for  instance,  the  case  of  persons  who  are  called 
red-blind  and  green-blind  on  the  Young  theory.  "We 
are  told  by  the  Hering  theory  that  both  are  red-green- 
blind — that  is,  blind  to  both  green  and  red,  and  only 
see  blue  and  yellow — and  that  the  only  difference  be- 
tween them  is  that  the  former  has  his  spectrum  slightly 
shortened  at  the  red  end,  the  maxima  of  the  yellow- 
blue  sensations  being  shifted  a  little  further  towards 
the  violet  end  of  the  spectrum.  The  natural  question 
to  ask  is  :  Why  this  shift  occurs  ?  Surely  it  is  more 
rational  to  adopt  a  theory  w^hich  does  not  require  such  a 
supposition  ?  If  the  sensitive  matter  acted  upon  by  the 
yellow-blue  rays  be  always  of  the  same  chemical  com- 
position, the  shift  cannot  occur.  It  might,  perhaps, 
be  allowed  that  one  shift  was  practicable,  but,  unfor- 
tunately, the  shifts  must  become  numerous  when  the 
cases  of  partial  colour  blindness  are  to  be  accounted 
for,  and  this  would  necessitate  a  constantly  varying 
chemical  composition  of  this  matter,  and  of  that  acted 
upon  by  the  red-green  rays. 

Again,  in  the  extinction  of  the  spectrum,  the  red  and 
the  green  sensations  in   quantities    to   neutralize    one 


You7ig  V.  Hering.  193 

another  sliould  be  extinguished  nearly  together,  even 
allowing  for  what  physiologists  tell  us  is  the  case,  that 
the  breaking  down,  or  dissimulation,  of  cell  tissue  con- 
tinues longer  than  its  building  up,  but  we  find  a  large 
difference  between  the  two.  As  already  indicated,  the 
luminosity  curve  of  the  feeble  spectrum  favours  the 
theory  of  Hering  being  that  here  we  only  have  the 
white-black  sensation,  and  naturally  the  persistency 
curves  must  be  scored  in  its  favour.  But  the  cases  of 
B.  C.  and  M.,  it  seems  to  me,  cannot  be  explained  by 
the  theory  without  any  undue  straining  or  assumptions. 
If  we  try  and  fit  the  cases  of  colour  blindness  due  to 
tobacco  scotoma  to  the  theory,  we  find  that  in  many 
cases  yellow  is  not  recognised,  though  blue  is  invariably. 
If  the  blue  be  active,  the  yellow  should  also  be  so. 

And  here  I  may  remark  that  it  has  been  assumed 
that  the  two  classes  of  colour  blindness  are  due  to 
difi'erent  causes.  A  question  to  ask  ourselves  is  whether 
all  colour  blindness  may  not  have  been  caused  originally 
by  disease.  In  the  congenital  form,  it  is  true,  no  disease 
of  the  retina  is  traceable  in  the  eye,  and  it  is  usually 
hereditary,  but  it  does  not  follow  that  the  want  of 
response  of  the  perceiving  apparatus  to  certain  sensa- 
tions may  not  have  been  due  to  what,  for  want  of  a 
better   expression,    I   may   call    an    hereditary   partial 


194  Colour  Vision. 

paralysis  of  the  perceiving  apparatus.  If  this  be  so, 
we  have  a  connecting  link  between  the  two  classes,  and 
then  a  perfect  theory  should  explain  both  classes  on  the 
same  grounds.  The  suspicion  that  the  monochromatic 
vision  of  P.  and  Q.  might  possibly  be  due  to  disease 
before  birth,  owing  to  the  behaviour  of  their  eyes  under 
certain  conditions,  would  then  be  explicable.  I  have 
no  desire  to  press  this  view,  though  it  seems  to  me  to 
be  one  which  is  not  out  of  all  reason,  taking  analogies 
from  other  defects  which  are  hereditary. 

It  has  been  usually  accepted  that  the  fields  for  blue 
and  yellow  in  the  eye  are  approximately  the  same,  as 
are  those  of  the  green  and  red,  and  this  has  been  taken 
as  showing  the  interdependence  between  the  two  pairs 
according  to  the  Hering  theory.     It  has  already  been 
pointed  out  that  the   question  of  extent  of  fields  re- 
quires  still   further   investigation    beyond   that  which 
it  has   received,    and   measures  made   by  the   method 
given   on    page    208    seem    to    cast    a    doubt    as    to 
whether  this  interdependence  can  be  upheld.     It  will 
be  noticed  that  the  fields  do  not  extend  proportion- 
ately on  the  nasal  and  temporal  sides  (see  also  Fig.  3). 
It  should  also  be  remarked  that  the  order  of  extent 
of  field  for  the  different  colours  does  not  follow  the 
same  order  as  their  disappearance.      A  point  that  is 


Young  V.  Hering.  195 

sometimes  raised  in  favour  of  Hering's  theory  is  the 
negative  image  formed  after  the  eye  is  fatigued  by 
hooking  at  bright  red  or  bright  green.  The  negative 
images  (see  page  30)  are  said  to  be  the  complementary 
of  these  colours.  The  Young  theory  tells  us '  that  the 
red  or  the  green  sensation  suffers  fatigue  by  one  or 
other  colour,  and  that  when  the  eye  subsequently  rests 
on  a  grey  surface  the  other  two  sensations  are  chiefly 
stimulated  and  cause  the  complementary  colour.  It  is 
said  that  it  is  easier  to  produce  a  negative  green  image 
than  a  negative  red  image,  and  the  adherents  of  Hering 
tell  us  that  this  is  due  to  the  fact  that  destructive 
action  is  more  readily  carried  out  than  constructive. 
In  the  Young  theory,  it  is  held  that  the  green 
sensation  is  always  mixed  with  white,  whilst  the  red 
is  fairly  pure,  and  thus,  for  equal  luminosities,  the 
surplus  green  sensation  is  much  less  stimulated  than 
the  red,  which  offers  a  consistent  explanation  of  this 
fact.  There  are  several  other  minor  difficulties  in  the 
way  of  accepting  Hering's  theory  as  it  stands  from  a  phy- 
sical point  of  view,  but  we  need  not  discuss  them  now. 

The  final  sensation  curves  for  the  spectrum  colours 
on  the  Young  theory  are  still  under  consideration,  and 
are  not  definitely  fixed,  though  the  observations  made 
have  been  very  numerous.     Eecently  Helmholtz,  in  the 

0  2 


196  Colour  Vision, 

last  edition  of  his  "  Physiological  Optics,"  has  calcu- 
lated, from  Koenig's  observations,  that  no  one  of  the 
three  sensations  is  singly  stimulated  by  any  colour, 
even  at  the  extreme  ends  of  the  spectrum,  and  he 
makes  the  three  fundamental  sensations  vary  consider- 
ably from  those  given  in  these  pages.  Every  colour 
he  states  is  considerably  mixed  with  white  light.  The 
calculations  by  which  he  arrived  at  this  conclusion  are 
of  a  complicated  nature,  and  I  think  if  he  had  had 
besides  the  colour  equations  of  Koenig,  the  luminosities 
and  the  extinction  measures  before  him,  there  might 
have  been  a  modification  of  his  views,  for  these  last 
give  evidence  to  the  contrary. 

There  is  a  possible  modification  of  the  Young  theory 
which  would  account  for  a  good  many  of  the  pheno- 
mena that  are  unaccounted  for  by  it  in  its  present 
form,  though  it  may  raise  new  difficulties  in  the  minds 
of  some.  Let  us  suppose  that  each  of  the  three  sensa- 
tions were  compounded  of  fundamental  liglit  and  of 
colour  in  fixed  and  definite  proportions,  and  not  in 
the  same  proportion  in  each ;  and  further  that  the 
apparatus  in  the  eye  which  was  responsible  for  each 
sensation  had  two  functions,  one  of  which  was  to 
respond  to  the  fundamental  light  sensation  and  the 
other  to  the  colour.     One  essential  difference  between 


Modified  Young  Theory.  197 

this  modification  of  the  Young  theory  and  that  of 
Hering  is  that,  whilst  in  the  latter  the  white  sensation 
is  a  sensation  distinct  from  the  colour  sensations,  in  the 
former  it  is  a  definite  part  of  them.  The  fact  that  the 
sensation  of  colour  is  lost  before  the  sensation  of  light 
is  one  of  the  greatest  significance,  and  any  theory 
to  be  accepted  must  offer  a  reasonable  explanation  of 
it.  If  the  modification  suggested  be  made,  it  accounts 
for  the  existence  of  this  residuum  of  light  equally 
as  well  as  Hering's  theory,  and  without  its  drawback. 
It  is  not  hard  to  imagine  the  apparatus  which  gives  rise 
to  two  sensations,  on  the  assumption  of  different  kinds 
of  atomic  motion,  induced  by  the  ether  motion,  or  at 
least  three  kinds  are  possible.  When  extinction  of 
colour  is  made,  the  ether  vibrations  would  have  suffi- 
cient energy  to  induce  but  one  kind  of  motion  ;  and 
when  all  light  was  extinguished  from  the  same  ray,  they 
would  not  be  capable  of  inducing  any  sensible  motion 
whatever.  In  the  case  of  Miss  W.,  who  saw  all  colours 
as  white,  it  might  be  that  disease  had  entirely  prevented 
the  first  kind  of  motion  in  all  three  sensations,  and 
that  in  P.  and  Q.  the  red  and  green  sensations  were 
absent  or  paralysed  in  their  entirety,  whilst  the  blue 
sensation  was  left  in  full  operation.  In  B.  C.  the  blue 
and  red  sensations  would  be  similarly  absent,  leaving 


igS  ColoM r   Vision . 

the  green  sensation  unchanged.  The  coincidence  of 
their  persistency  and  luminosity  curves  would  then 
indicate  that  the  proportions  of  fundamental  light  and 
colour  remained  the  same  throughout.  Other  examples 
and  considerations  seem  to  indicate  that  the  proportion 
of  colour  to  fundamental  light  is  greatest  in  the  red 
sensation,  next  in  the  green,  and  least  in  the  blue. 
This  would  explain  why  with  increasing  intensities  blue 
appears  white  sooner  than  green,  and  much  sooner 
than  red.  The  proposed  modification  would  also  offer 
the  necessary  explanation  as  to  the  disappearance  of 
colour  from  the  field. 

Looking  at  colour  vision  from  what  I  may  call  an 
evolutionary  point  of  view,  the  "  light-colour  "  theory 
commends  itself  as  probable.  There  are  many  reasons 
for  thinking  that  the  visual  sensation  first  evolved  was 
that  of  light,  subsequently  followed  by  that  of  colour. 
The  first  evolved  colour  sensation  would  appear  to  have 
been  the  blue,  and  the  last  the  red.  The  discussion  of 
this  hypothesis  would  carry  me  beyond  my  limits,  and 
I  must  leave  it  thus  baldly  expressed  for  your 
consideration. 

For  my  own  part,  whatever  theory  of  colour  sensations 
may  prove  to  be  the  right  one,  I  lean  strongly  to  the 
idea  that  the  cause  of  vision  will  be  found  in  chemical 


Colour  Sensation  and  Photographic  Action.    199 

actioD,  induced  by  the  impact  of  the  different  wave- 
lengths of  light  falling  on  sensitive  matter.  A  white 
substance  may  absorb  all  the  wave-lengths  found  in  the 
spectrum,  and  if  it  have  three  sets  of  molecules,  one  of 
which  has  an  atom  or  atoms  vibrating  with  the  same 
period  as  the  waves  of  light  which  show  a  maximum 
for  one  sensation  and  another  for  another,  and  so  on,  the 
requirements  for  the  colour  sensations  are  met.  It  may 
be  that  the  sensitive  part  of  the  retina  is  like  a  photo- 
graphic plate,  but  with  this  essential  difference — that  the 
sensitive  material  is  constantly  changing.  A  photo- 
graphic plate  receives  an  impression  which  is  not 
recognisable  by  the  eye,  though  it  can  be  shown  that  a 
change  in  the  material  does  take  place  during  the 
impact  of  light,  by  electrical  and  other  means.  When 
the  eye  receives  an  impression  of  light,  Dewar  has 
shown  that  in  this  case  also  a  current  of  electricity  is 
generated.  Kecent  published  experiments  of  my  own 
have  demonstrated  that  with  a  low  intensity  of  light, 
the  chemical  change  that  occurs  in  a  photographic  salt 
is  by  no  means  proportionate  to  that  which  takes  place 
with  a  greater  intensity.  In  the  eye,  too,  there  is  a 
limit  of  sensibility  to  very  feeble  light.  Again,  the 
curves  of  the  stimulation  of  the  colour  sensations  to  the 
spectrum  are  closely  of  the  same  form  as  the  curves 


200  Colour   Vision. 

of  sensitiveness  of  the  various  sensitive  salts  used  by 
photographers.  These  are  analogies  and,  of  course, 
must  not  be  pressed  too  far.  There  must  be  such  a 
complexity  in  the  sensitive  material  in  the  eye,  both 
chemical  and  physiological,  that  it  may  be  that  the 
changes  induced  by  light  on  the  sensitive  surface  of  the 
retina  have  to  be  considered  from  both  aspects.  The 
purely  chemical  change  is  naturally  that  to  which  a 
physicist  is  most  prone  to  incline,  and  his  bias  must 
be  discounted,  as  must  also  that  of  the  physiologist. 


(   ^oi   ) 


APPENDIX. 

The  following  is  extracted  from  Maxwell's  paper. 

The  following  table  contains  the  means  of  four  sets 
of  observations  by  the  same  observer  (K.)  : — 

Table  IV.     (K.) 

44-3  (20)  4-31-0  (44)  +  27-7  (68)  =  W. 
16-1  (28)  +  25-6  (44)  +  30-6  (68)  =  W. 
22-0  (32)  4-  12-]  (44)  +  30-6  (68)  =  W. 
6-4  (24)  +  25-2  (36)  +  31-3  (68)  =  W. 
15-3  (24)  +  26-0  (40)  +  30-7  (68)  =  W. 
19-8  (24)  4-35-0  (46)  4-30-2  (68)  =  W. 
21-2  (24)  +  41-4  (48)  4-  27-0  (68)  =  W. 
22-0  (24)  4-  62-0  (52)  4-  13-0  (68)  =  W. 
21-7  (24)  4-  10-4  (44)  4-  61-7  (56)  =  W. 
20-5  (24)  +  23-7  (44)  4-  40-5  (60)  =  W. 
19-7  (24)  +  30-3  (44)  +  33-7  (64)  =  W. 
18-0  (24)  +  31-2  (44)  4-  32-3  (72)  =  W. 
17-5  (24)  +  30-7  (44)  4-  44-0  (76)  =  W. 
18-3  (24;  +  33-2  (44)  +  63-7  (80)  =  W. 

X. — Keduction  of  the  Observations. 

By    eliminating   W  from    the    equations;  above    hy 
means    of  the  standard  equation/^^\>fc^^^^^4^ 


.^^ 


202  Appendix, 

involving  each  of  the  fourteen  selected  colours  of  the 
spectrum,  along  with  the  three  standard  colours  ;  and 
by  transposing  the  selected  colour  to  one  side  of  the 
equation,  we  obtain  its  value  in  terms  of  the  three 
standards.  If  any  of  the  terms  of  these  equations  are 
negative,  the  equation  has  no  physical  interpretation  as 
it  stands  ;  but  by  transposing  the  negative  term  to  the 
other  side  it  becomes  positive,  and  then  the  equation 
may  be  verified. 

The  following  table  contains  the  values  of  the  four- 
teen selected  tints  in  terms  of  the  standards.  To  avoid 
repetition,  the  symbols  of  the  standard  colours  are 
placed  at  the  head  of  each  column  :  — 


Table  VI. 

Observer  (K.) 

(24) 

(44) 

(G8) 

44-3  (20)  = 

18-6 

+     0-4 

+     2-8 

16-1  (28)  = 

18-6 

+     5-8 

-     0-1 

22-0  (32)  = 

18-6 

+  19-3 

-    0-1 

25-2  (36)  = 

12-2 

+  31-4 

-     0-8 

26-0  (40)  = 

3-3 

+  31-4 

-     0-2 

35-0  (46)  = 

-     1-2 

+  31-4 

+    0-3 

41-4  (48)  = 

-     2-6 

+  31-4 

+     3-5 

62-0  (52)  = 

-     3-4 

+  31-4 

+  17-5 

61-7  (56)  = 

-     3-1 

+  21-0 

+  30-5 

40-5  (60)  = 

-     1-9 

+     7-7 

+  30-5 

33-7  (64)  = 

-     1-1 

+     1-1 

+  30-5 

32-3  (72)  = 

+     0-6 

+     0-2 

+  30-5 

44-0  (76)  = 

+     1-1 

+     0-7 

+  30-5 

63-7  (80)  = 

+    0-3 

-     1-8 

+  30-5 

Appendix,  203 

Mr.  James  Simpson,  formerly  student  of  Natural 
Philosophy  in  my  class,  has  furnished  me  with  thirty - 
three  observations  taken  in  good  sunlight.  Ten  of 
these  were  between  the  two  standard  colours,  and  give 
the  following  result  :— 

33-7  (88)  +  33-1  (68)  =  W. 

The  mean  errors  of  these  observations  were  as 
follows  : — 

Error  of  (88)  =  2  •  5  ;    of  (68)  =  2  •  3  ;    of  (88)  +  (68) 
=  4-8  ;  of  (88)  -  (68)  =  1-3. 

The  fact  that  the  mean  error  of  the  sum  was  so 
much  greater  than  the  mean  error  of  the  difference, 
indicates  that  in  this' case,  as  in  all  others  that  I  have 
examined,  observations  of  equality  of  tint  can  be 
depended  on  much  more  than  observations  of  equality 
of  illumination  or  brightness. 

From  six  observations  of  my  own,  made  at  the  same 
time,  I  have  deduced  the  "trichromic"  equation — 

22-6  (104)  +  26  (88)  +  37-4  (68)  =  W   ...    (2) 

If  we  suppose  that  the  light  which  reached  the 
organ  of  vision  was  the  same  in  both  cases,  we  may 
combine  these  equations  by  subtraction,  and  so  find 

22-6  (104)  -  7-7  (88)  +4-3  (68)  =  D.    .    .    .   (3) 


204  Appendix, 

where  D  is  that  colour,  the  absence  of  the  sensation  of 
which  constitutes  the  defect  of  the  dichromic  eye. 

The  sensation  w^hich  I  have  in  addition  to  those  of 
the  dichromic  eye  is  therefore  similar  to  the  full  red 
(104),  but  different  from  it  in  that  the  red  (104)  has 
"7 '1  of  green  (88)  in  it  which  must  be  removed,  and 
4*3  of  blue  (68)  substituted.  This  agrees  pretty  w^ell 
w^ith  the  colour  which  Mr.  Pole*  describes  as  neutral 
to  him,  though  crimson  to  others.  It  must  be  re- 
membered, however,  that  different  persons  of  ordinary 
vision  require  different  proportions  of  the  standard 
colours,  probably  owing  to  differences  in  the  absorptive 
powers  of  the  media  of  the  eye,  and  that  the  above 
equation  (2),  if  observed  by  K.,  would  have  been 

23(104)  +  32  (88)  +  31  (68)  =  W (4) 

and  the  value   of  D,  as   deduced  from  these  observers, 
would  have  been 

23  (104)  -  1-7  (88)  -~  1-1  (68)  =  D  .  .  .  .  (5) 
in  which  the  defective  sensation  is  much  nearer  to  the 
red  of  the  spectrum.  It  is  probably  a  colour  to  which 
the  extreme  red  of  the  spectrum  tends,  and  which 
differs  from  the  extreme  red  only  in  not  containing  that 
small  proportion  of  "  yellow  "  light  which  renders  it 
visible  to  the  colour  blind. 

*  PLilosophical  Transactions,  1859,  Part  I.,  p.  329. 


Appendix.  205 

From  other  observations  by  Mr.  Simpson  tbe  follow- 


ino;  results  have  been  deduced  : — 


Table  A. 

(88) 

(68) 

(88) 

(68) 

(99-2  +)  = 

33-7 

1-9 

100  (96)  = 

108 

7 

31-3  (96)  = 

33-7 

2-1 

100  (92)  = 

120 

5 

28   (92)  = 

33-7 

1-4 

100  (88)  = 

100 

0 

33-7  (88)  = 

33-7 

0 

100  (84)  = 

61 

11 

54-7  (84)  = 

33-7 

6-1 

100  (82)  = 

47 

21 

71   (82)  = 

33-7 

15-1 

100  (80)  = 

34 

33 

99   (80)  = 

33-7 

33-1 

100  (78)  = 

22 

47 

70   (78)  = 

15-7 

33-1 

100  (76)  = 

10 

59 

56   (76)  = 

5-7 

33-1 

100  (72)  = 

1 

92 

36   (72)  = 

0-3 

33-1 

100  (68)  = 

0 

100 

33-1  (68)  = 

0 

33-1 

100  (64)  = 

0 

83 

40   (64)  = 

0-2 

33-1 

100  (60)  = 

3 

60 

55-5  (60)  = 

1-7 

33-1 

57-)    = 

0-3 

33-1 

i 

In  the  table  on  the  left  side  (99*2  +)  means  the 
whole  of  the  spectrum  beyond  (99*2)  on  the  scale,  and 
(57 — )  means  the  whole  beyond  {bl)  on  the  scale.  The 
position  of  the  fixed  lines  with  reference  to  the  scale 
was  as  follows  : — 

A,  116  ;  a,  112  ;  B,  110  ;  C,  106  ;  D,  98*3  ;  E,  88  ; 
F,  79  ;  Gl,  61  ;  H,  44. 

The  values  of  the  standard  colours  in  different  parts 
of  the  spectrum  are  given  on  the  right  side  of  the 
above    table,    and    are   represented   by   the   curves   of 


2o6  Appendix. 

Fig.  9,  Plate  II.,  where  the  left-hand  curve  represents 
the  intensity  of  the  ''  yellow  "  element,  and  the  right- 
hand  curve  that  of  the  "  blue  "  element  of  colour  as  it 
appears  to  the  colour  blind. 

The  appearance  of  the  spectrum  to  the  colour  blind 
is  as  follows  : — 

From  A  to  E  the  colour  is  pure  "  yellow,"  very  faint 
up  to  D,  and  reaching  a  maximum  between  D  and  E. 
From  E  to  one-third  beyond  F  towards  G  the  colour  is 
mixed,  varying  from  "  yellow  "  to  "  blue,"  and  becoming- 
neutral  or  "  white  "  at  a  point  near  F.  In  this  part  of 
the  spectrum  the  total  intensity,  as  given  by  the  dotted 
line,  is  decidedly  less  than  on  either  side  of  it,  and  near 
the  line  F,  the  retina  close  to  the  "  yellow  spot "  is  less 
sensible  to  light  than  the  parts  further  from  the  axis 
of  the  eye.  This  peculiarity  of  the  light  near  F  is 
even  more  marked  in  the  colour  blind  than  in  the 
ordinary  eye.  Beyond  F  the  "  blue  "  element  comes  to 
a  maximum  between  F  and  G,  and  then  diminishes 
towards  H,  the  spectrum  from  this  maximum  to  the 
end  being  pure  "  blue." 

The  results  given  above  wxre  all  obtained  with  the 
light  of  white  paper,  placed  in  clear  sunshine.  I  have 
obtained  similar  results  when  the  sun  was  hidden,  by 
using  the  light  of  uniformly  illuminated  clouds,  but  I 


Appendix.  207 

do  not  consider  these  observations  sufficiently  free  from 
disturbing  circumstances  to  be  employed  in  calculation. 
It  is  easy,  however,  by  means  of  such  observations,  to 
verify  the  most  remarkable  phenomena  of  colour 
blindness,  as,  for  instance,  that  the  colours  from  red  to 
green  appear  to  differ  only  in  brightness,  and  that  the 
brightness  may  be  made  identical  by  changing  the 
width  of  the  slit ;  that  the  colour  near  F  is  a  neutral 
tint,  and  that  the  eye  in  viewing  it  sees  a  dark  spot  in 
the  direction  of  the  axis  of  vision  ;  that  the  colours 
beyond  are  all  blue  of  different  intensities,  and  that 
any  "  blue  "  may  be  combined  with  any  "  yellow  "  in 
such  proportions  as  to  form  "  white."  These  results  I 
have  verified  by  the  observations  of  another  colour- 
blind gentleman,  who  did  not  obtain  sunlight  for  his 
observations  ;  and  as  I  have  now  the  means  of  carrying 
the  requisite  apparatus  easily,  I  hope  to  meet  with 
other  colour-blind  observers,  and  to  obtain  their  obser- 
vations under  more  favourable  circumstances. 

Measurements  of  Colour  Fields. 

Some  experiments  in  the  measurement  of  the  colour 
fields  in  the  horizontal  direction  with  the  pure  spectrum 
colours  will  help  to  show  what  importance  is  to  be  attached 


2o8  Appendix, 

to  the  luminosity  of  the  colour  and  the  size  of  the  spot  of 
light  with  which  the  observations  are  made.  A  yellow 
and  a  blue  of  the  spectrum  were  taken  of  such  hues 
that  when  mixed  they  formed  a  patch  of  white  light 
similar  to  the  electric  light.  Their  luminosities 
were  measured,  and  the  yellow  found  to  be  1  •  6  of  the 
light  of  an  amyl-acetate  lamp  or  1*28  standard  candles  ; 
the  blue  was  -^^  of  this  luminosity.  The  fields  for  these 
two  colours  were  measured  by  automatically  throwing 
spots  of  each  colour  separately  on  a  white  card  which 
moved  round  a  centre  over  which  the  eye  was  placed. 
The  light  was  subsequently  diminished  to  \,  \,  and  -§- 
of  the  above  values,  and  readings  again  made.  The 
following  results  were  obtained  with  a  spot  of  •  7  inch 
diameter  : — 

Yellow.  Blue. 


Light. 

Nasal  side. 

Temporal  side. 

Nafcal  side. 

Temporal  side. 

Full 

33° 

45° 

35° 

45° 

i- 

24° 

36° 

26° 

38° 

i 

18° 

24° 

22° 

32° 

With  a  spot  of  •  3  inch  diameter  the  following  were 
obtained  : — 


Qll 

24° 

32° 

21° 

27° 

i 

17° 

28° 

16° 

22° 

i 

13° 

16° 

14° 

.    20° 

i 

8° 

10° 

13° 

1G° 

Appendix,  ^2,09 

It  will  ■  be  evident  how  the  field  contracts  as  the 
light  is  diminished  in  brightness,  and  also  that  the  blue 
field  does  not  diminish  equally  with  the  yellow  field, 
but  is  more  persistent.  Again,  it  will  be  noticed 
that  the  luminosity  of  the  blue,  for  the  same  extent  of 
field  to  be  covered,  has  to  be  much  lower  than  for 
the  yellow. 

The  diminished  area  of  the  spot  of  light  also 
diminishes  the  field,  and  the  same  order  of  diminution 
of  field  is  obtained  as  with  the  larger  spot. 

Another  set  of  experiments,  made  with  the  same 
aperture  of  slit  passed  through  the  spectrum,  and  the 
field  taken  at  difi*erent  points,  give  the  following 
results  : — 


Spectrum  scale. 
(See  Fig.  41,  page  210.) 

58-6 

o4-rt 

50-6 

46-6 

42-6 

38-6 

34-6 

30-6 

26-0 

22-G 

.18*6 

14-6 

8-6 


'asal  side. 

Temporal  side. 

18°       • 

35° 

27° 

46° 

88° 

47° 

2o° 

30° 

21° 

21° 

17° 

17° 

22° 

30° 

25° 

33° 

33° 

40° 

37° 

44° 

28° 

40° 

22° 

34° 

20° 

30° 

2IO 


Appendix, 


Here   we   see  that  although  the  luminosity  of  the 
colour    spots  varies    at  the    spectrum    luminosity,    the 


Fig.  41. 


Horizontal  Field  for  spectrum  colours 


Li        F  E 


D  CLt 


44      48       52       56      60 


jectrum   Scale 


fields  do  not  vary  proportionally ;  when  the  lumin- 
osities of  the  green,  yellow  and  red  are  made  equal,  the 
fields  become  nearly  equal  on  the  nasal  side.  The 
field  for  the  blue,  however,  then  becomes  vastly  larger 
than  that  for  the  others,  showing  a  peculiarity  which 
is  very  remarkable. 

Eecently  published  experiments  on  colour  fields 
have  been  so  largely  based  on  the  exigencies  of  the 
Hering  theory,  that  it  is  somewhat  difficult  to  decide 
their   significance  from  any  other  aspect. 


Appendix. 


2l\ 


Table  I. — Luminosity  Cueves  for  the  Normal  Eye  (see  Fig.  20). 


I. 

11. 

III. 

IV. 

_        V. 

Scale 
number. 

Wave-length. 

Outside  yellow 
spot. 

Yellow  spot. 

Fovea 
centralis. 

64 

7217 

' 

63    : 

7082 

1 

62 

6957 

1 

2 

2 

61 

6839 

2 

4 

4 

60  • 

6728 

3-5 

7 

8. 

59 

6621 

7-5 

12-5 

15-5 

58, 

6520 

12-5 

21 

24 

57 

6423 

19 

33 

37:5 

56 

6330 

27-5 

50 

60. 

55 

6242 

35 

65 

77 

54 

6152 

43 

80 

90, 

53 

6074 

52-5 

90 

97. 

52 

5996 

61-0 

96 

100 

51 

5919 

71-0 

99 

100, 

50 

5850 

79-0 

100 

98 

49 

5783 

84 

99 

95, 

48 

5720 

85 

97 

90. 

47 

5658 

83-5 

92-5 

i     85 

46 

5596 

81-0 

87 

79 

45 

5538 

77-0 

81 

72:5 

44 

5481 

72.5 

75 

66 

43 

5427 

68-0 

69 

59 

42 

5373 

62.5 

62-5 

51 

41 

5321 

57 

57 

45 

P    2 


212 

Appendix, 
Table  I. — continued. 

I. 

II. 

III. 

IV. 

V. 

Scale 
number. 

Wave-length. 

Outside  yellow 
spot. 

Yellow  spot. 

Fovea 
centralis. 

40 

5270 

52 

50 

40 

39 

5221 

46 

42-5 

32 

38 

5172 

41-5 

36 

27-5 

37 

5128 

37-5 

29-5 

22^0 

36 

5085 

33-5 

24 

18 

35 

5043 

30-0 

18-2 

14 

34 

6002 

26-5 

14-2 

10 

33 

4963 

24 

10-5 

8^4 

32 

4924 

21 

8-5 

6-5 

31 

4885 

18-5 

7-0 

5-5 

30 

4848 

16-5 

5-5 

4-0 

29 

4812 

14-5 

4-7 

3^5 

28 

4776 

13-0 

4-0 

3^0 

27 

4742 

11-5 

3-5 

2^0 

26 

4707 

10.5 

2-8 

2^4 

25 

4675 

9-4 

2-3 

2^1 

24 

4639 

8-2 

1-82 

1-9 

23 

4608 

7-3 

1-6 

1^5 

22 

4578 

6-3 

1-4 

21 

4548 

5-7 

1-2 

20 

4517 

5-0 

1.08 

1-0 

19 

4488 

4-5 

•94 

18 

4459 

4-0 

•86 

17 

4437 

3-6 

•78 

16 

4404 

3-1 

•70 

Appendix, 


2I3r 


f 

Pable  I. — continued 

i: 

II. 

III. 

IV. 

V. 

Scale 
number. 

"Wave-length. 

Outside  yellow 

spot. 

Yellow  spot. 

Fovea 
centralis. 

15 

14 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

4377 
4349 
4323 
'  4296 
4271 
4245 
4221 
4197 
4174 
4151 
4131 
4106 

2-7 

2-3 

2-1 

1-9 

1-65 

1-4 

1-2 

1-0 
•88 
•75 
•63 
•50 

•62 
•56 
•50 
■45 
•40 
•34 

30 
•26 

22 
•18 

16 

14 

•62 

:, 

^-' 

<'  c* 

C^ 


M4 


Appendix, 


Tables  II.  Lm>  1X1. — Curves  of  Luminosity  of  a  Partially 
Eed-Blind  and  of  a  Partially  Green-Blind  Person  (see 
Fig.  23). 


Luminosity, 

Scale  number* 

Wave-length. 

Red-blind. 

Green-blind. 

64 

7217 

0 

1 

62 

6957 

1 

2 

60 

6728 

2 

7 

68 

6520 

6 

21 

66 

6330 

12 

50 

64 

6152 

26 

80 

62 

5996 

49 

96 

60 

5850 

70 

98 

48 

5720 

77 

93 

46 

5596 

77 

83 

44 

-        5481 
6373 

70 

70 

42 

1  \r 

61 

55 

40 

6270 

47 

40 

38 

6172 

34 

27 

36 

6086 

23 

18 

34 

6002 

14 

10 

32 

4924 

8-5 

5-5 

30 

4848 

5-5 

3*0 

28 

4776 

4-0 

2-5 

26 

4707 

2-7 

2-0 

24 

4639 

1-8 

1*8 

22 

4678 

1-35 

1-4 

20 

4517 

1-1 

la 

Appendix. 


21 


Table   IV. — Luminosity   of   SrEcxRUM   Eeduced  in   Intensity,  so 
THAT  D  =  ^y^^.  Amyl  Lamp  1  Foot  Distant  (see  Fig.  25). 


Scale 
numberi 

Mean 
reading. 

55-6 

.      *5 

53-6 

5-5 

51-6 

13 

49-6 

23 

47-6 

40 

45-6 

57 

43-6 

70 

41-G 

79 

39-6 

78 

37-6 

74 

35-6 

^^ 

33-6 

65 

31-6 

44-5 

29-6 

35 

27-6 

24 

25-6 

17 

23-6 

13 

21-6 

10 

19-6 

8 

13-6 

3 

9-6 

2 

Mean  rcadin* 
reduced  to 
100  max. 


•6 
7-0 
lG-7 
29-7 
50-0 
71-2 
87-5 
98-7 
97-5 
92-5 
82-5 
68-7 
55-2 
43-7 
30-0 
21-7 
16-7 
12-5 

io-{r 

3-7 
2-5 


P.  and  Q.' 

readings, 
100  max. 


2 

3-6 

8 

22 

44 

69 

93 
100 

99-5 

96 

89 

77-5 

61 

45*5 

33-5 

25 

18 

13 
9-5 
4-2 
2-5 


Persistency- 
curve  for  the 
centre  of 
the  eye. 


2 

3-6 

8 
22 
44 
69 
93 

99-5 
98-5 
93 
84 
71 

63»5 
36-5 
24 
16 
10 

8 

6 

3 

2 


2i6  Appendix, 

Table  V. — Limit  of  Colour  Vision  (see  Fig.  26.) 


Scale 
Number. 


Wave- 
Length. 


Mean  reading 
of  the  colour 
limit  of  the 
spectrum  D. 

being  1  amyl 
lamp  in  T^gths. 


Luminosity  of 

the  ordinary 

spectrum. 


Luminosity  of 
the  rays  when 
each  colour  dis- 
appears, each 
ray  having 
the  original 
luminosity  of 
1  amyl  lamp 


61 

6839 

120  . 

4 

48-0 

60 

6728 

67 

'     7 

46^9 

58 

6520 

26 

21 

54^6 

56 

6330 

13 

50 

65-0 

54 

6152 

9-5 

80 

76-0  - 

52 

5996 

9-0 

96 

86-4 

50 

5850 

9-0 

100 

90^0 

48 

5720 

9-0 

97 

87^3 

44 

5481 

9-5 

75 

71-3 

40 

5270 

10-5 

50 

52-5 

36 

5085 

12-5 

24 

30-0 

32 

4924 

18 

8.5' 

15^3 

28 

4776 

32 

4-0 

12-8 

24 

4639 

55 

1-8  - 

12-0 

20 

4517 

90   . 

1-08 

9^7 

16 

4404 

160 

•70 

11-2 

12 

4296 

250 

•45 

11-0 

8  , 

4197 

400 

•26 

10/4 

4  ^ 

4106 

700 

•14  , 

9;8  , 

Appendix, 


2XT- 


Table  VI. — Extinction  by  Central  Portion  of  Normal  Eye 
(see  Fig.  28). 


I. 

II. 

III. 

IV. 

V. 

VI. 

Scale 
number. 

Wave- 
lengtli. 

E. 

Reduction  of 

original 

luminosity 

in  milliontliB 

to  cause 

extinction. 

L. 

Lumino- 
sity of 

original 
beam. 

ExL 

100  • 

Persistency 

curve 

650 

E 

(Maximum  = 

100). 

64 

7217 

55,000 

63 

7082 

30,000 

1 

300-0 

62 

7957 

15,000 

2 

300-0 

61 

6839 

7500 

4 

300-0 

60 

6728 

3750 

7 

262-5 

59 

6621 

1900 

12-5 

237-5 

•34 

58 

6520 

1050 

21 

220-5 

•62 

57 

6423 

650 

33 

214-5 

1-0 

56 

6333 

380 

50 

190-0 

1.71 

55 

6242 

272 

65 

176-8 

2-38 

54 

6152 

196 

80 

156-0 

3-32 

53 

6074 

140 

90 

126-0 

4-64 

52    ^ 

5996 

97 

96 

93-12 

6-70 

51 

5919 

57 

99 

56-43 

11-40 

50 

5850 

35 

100 

35-0 

18-6 

49 

5783 

24 

99 

23-76 

.  27-1 

' 

48 

5720 

17 

97 

16.49    . 

...   38-2 

47    " 

5658 

12-6 

;92-5 

11'65 

:   51-6' 

46   ' 

5596 

10-2 

-87 

8-87   ' 

63-7 

45 

5538 

8-6 

81 

6-97  ; 

1      75-6 

441 

5481 

.        7-4 

75 

5-55  '<- 

r    87-81-; 

21 8 


Appendix, 

Table  YI.-^-*-continued. 


I. 

II. 

III. 

IV. 

V. 

VI. 

Scale 
number. 

Wave- 
length. 

E. 

Reduction  of 

original 

luminosity 

in  millionths 

to  cause 

extinction. 

L. 

Lumino- 
sity of 

original 
beam. 

ExL 

100.  ■ 

Persistency 

curve 

650 

E 

(Maximum  = 

100). 

43 

5427 

6-7 

69 

4-62 

97-0 

42 

5373 

6-55 

62-5 

4-09 

99-5 

41 

5321 

6-5 

57 

3-705 

100 

40 

5270 

6-55 

50 

3-27 

98^5 

39 

5221 

6-65 

42-5 

2.83 

97-5 

38 

5172 

6-85 

36 

2-46 

95-0 

SI 

5128 

- .   7-2      • 

29-5 

2-12 

90-0 

36 

5085 

7-6 

24 

1-82 

81-3 

35^ 

5043 

8-15 

18-2 

1-48 

80-0 

34- 

5002 

8-8 

14-2 

1-25 

74-0 

33 

4963 

10-2 

10-5 

.1-07 

63*0 

32 

4924 

11-6 

8-5 

•988 

56-0 

31 

4885 

^    13-6 

7-0 

•952 

47-7 

30. 

4848 

16-3 

5-5 

•896 

40*0 

29 

4812 

20-5 

4-7 

•963 

31-7    , 

28 

4776 

'    26-0 

4-0 

1-040 

25-0 

27  i  ^  . 

4742 

.      31-0 

3-5 

1-085 

20-9    . 

26 

4707 

38-5 

,2-8 

1-078 

16-9 

25u  ■  ■ 

>  4674  . 

".46-0 

2-3 

1-058 

14-1    . 

24 : 

4639 

-    56-0 

.    1-82 

1-019 

11*6 

23 

-    4608    . 

r    67-0 

.    1-6 

1072 

:    9-7,. 

22> 

4578 

.  ■    80 

1-4 

1-120 

8»41S 

Appendix. 
Table  VI. — continued. 


219 


I. 

II. 

III. 

IV. 

V. 

VI. 

Scale 
number. 

Wave- 
length. 

E. 

Bednction  of 

original 

luminosity 

in  millionths 

to  cause 
extinction. 

L. 

Lumino- 
sity of 

original 
beam. 

E  xL 

100.  * 

Persistency 

curve 

650 

E 

(Maximum  = 

100). 

21  . 

4548 

95 

1-2 

1^140 

7*22 

20 

4517 

107 

1-08 

1-156 

6-1 

19 

4488 

124 

•94 

1-165 

5-23 

18 

4459 

140 

•86 

1-204 

4-64 

17 

4437 

160 

•78 

1-228 

4-1 

16 

4404 

180 

•70 

1^260 

3^60 

15 

4377 

200 

•62 

1^240 

3^25 

14 

4349 

220 

*bQ 

1-232 

2*95 

13 

4323 

240 

•50 

1-200 

2^7 

12 

4296 

270 

•45 

1-215 

2^4 

11 

4271 

300 

•40 

1-200 

2-18 

10 

4245 

335 

•34 

1-139 

1-94 

9 

4221 

375 

•30 

1-125 

1-73 

8 

4197 

430 

•26 

1-118 

1^51 

T 

4174 

490 

•22 

1^078 

1-32 

6 

4151 

510 

•18 

•918 

1^27 

6 

4131 

640 

•16 

1-024 

1^01 

4- 

4106 

750 

•14 

1-050 

0-86 

220 


Appendix, 


Table  YII. — Extinction  by  Whole  Eye 
(see  Fig.  28). 


1. 

11. 

•       III. 

IV. 

V. 

VI. 

Scale    ' 
number. 

Wave- 
length. 

E. 

Reduction  of 

original 

luminosity 

in  millionths 
to  cause 
extinction. 

L. 

Luminosity 

of 

original 

beam. 

ExL 

■   160   • 

Persistency 

curve 

650 

E 

(Maximums 

100). 

38 

37 

36 

35      . 

34 

33 

32; 

31 

30    ,  . 
29    ,, 

28     ;. 

27,    . 

26;      . 

24: 
2^ 
20 
18 
16 

5172 
5128 
5085 
5043 
5002 
4963 
4924 
4885 
4848 
4812 
4776 
4742 
;    4707 
4639  : 
4578 
4517 
4459 
4404 

6-9 

7-1 

7-4 

7-7 

8-0 

8-4 

8-8 

9-4 

10-0 

10-7 

11-5 

13-0 

14-5 

18-5 

23-0 

30-0 

39-0 

51 

.    41-5 

.    37-5 

.    33-5 

,    30-0 

,    26-5 

24-0 

,    21-0 

-    18-5 

.    16-5 

,    14-5 

,    13-0 

.    11-5 

.    10-5 

,      8-2 

6-3 

5-0 

4-0 

3-1 

2-86 
2-66 
2-48 
2-31 
2-12 
2-02 
1-85 
1-74 
1-65 
1-55 
1-49 
1-49 
1-52 
1-52 
1-45 
1-50 
1-56 
1-59 

94-2 

91-6 

87-8 

84-4 

81-2 

77-5 

73-8 

69-2 

65-0 

60-6 

56*5^ 

50-0, 

44-8 

34-1 

28-3 

21-7 

16-7 

12-3 

Appendix. 


2T.I 


Table  YII. — continued. 


Scale 
number. 


14 
12 
10 
8 
6 
4 
2 
0 


II. 


Wave- 
length. 


4349 
4296 
4245 
4197 
4151 
4106 
4063 
4020 


III. 


E. 

Reduction  of 

originat 

luminosity 

in  millionths 

to  cause 

extinction. 


66 
80 
110 
154 
204 
307 
513 
770 


IV. 

V. 

L. 

Luminosity 

of 

original 

beam. 

ExL. 
160 

2-3 

1-52     1 

1-9 

1-52 

1-4 

1-54 

1-0 

1-54 

•75 

1-54 

•5 

1-54 

•3 

1-54 

•2 

1-54 

VI. 


Persistency 

curve 

650 

E 

(Maximum = 

100). 


9-85 
8-12 
5-91 
4-22 
3-18 
2-11 
1-26 
•84 


From  38  to  64  the  extinction  is  the  same  as  with  the  central 
part  of  the  eye. 


222 


Appendix. 
Table  VIIL— P.'s  Curves*  (see  Fig.  31). 


I. 

II. 

III. 

IV. 

V. 

VI. 

vn. 

Scale 
number. 

Wave- 
length. 

Mean 
reading  of 
extinction 

in 
millionths 
of  original 
luminosity. 

Adopted 
leading  in 
millionths 
of  original 
luminosity. 

Persistency 

curve 

680 

ad.  reading' 

P.'s 

luminosity 
curve. 

Absolute 
luminosity 

of 
extinction. 
IV.  X  VI. 

14 

I 
[ 

52 

5996 

68 

68 

10 

7 

34 

50 

5850 

35 

35 

19-4 

19 

47-5 

K 

5720 

17 

17 

40 

39 

47-3 

46 

5596 

10-2 

10 

68 

65 

46-4 

45 

5538 

9-3 

9-0 

76 

76 

48-8 

44 

5481 

8-0 

8-1 

84 

90 

52-8 

42 

5373 

7-2 

7-2 

94-5 

98 

50-3 

40 

5270 

6-7 

6-8 

100 

99 

48-1 

38 

5172 

7-2 

7-0 

97 

97-5 

48-7 

36 

5085 

8*05 

7-7 

90 

90 

49-5 

34 

5002 

8-05 

8-4 

81 

80 

47-9 

32 

4924 

9-9 

9-8 

69 

65 

45-5 

30 

4848 

13-2 

12-5 

54 

50 

44-6 

28 

4776 

13-9 

15-0 

45-3 

36 

38-6 

27 

4742 

16-8 

17-0 

40 

31-5 

38-2 

26 

4707 

21-6 

20-5 

32 

26-5 

38-8 

24 

4639 

30 

27 

25 

19-5 

37-6 

22 

4578 

36 

35 

19 

14 

35 

20 

4517 

42 

45 

15-5 

10 

32-2 

16 

4404 

79 

79 

8-5 

5-5 

31-2 

10 

4245 

180 

190 

3-6 

2-5 

32-2 

6 

4151 

270 

270 

2-7 

*  In  this  and  the  next  two  Tables  the  intensity  of  the  illumination  of  the  D 
ray  before  reduction  is  equal  to  that  of  an  amyl-acetate  lamp  at  one  foot  from  a 
screen.  The  figures  iu  Col.  VII.  are  in  millionths  of  the  illumination  of  an 
amyl-acetate  lamp  at  one  foot  distant,  every  ray  being  made  of  that  intensity. 


Appendix., 
Table  IX.— H.  R.'s  Curves  (see  Fig.  32). 


223 


J 


I. 

II. 

III. 

IV. 

V. 

VI. 

vir. 

Scale 
number. 

Wave- 
length. 

Mean 

reading  of 

extinction  in 

millionths  of 

original 
luminosity. 

Adopted 
reading  in 
millionths 
of  original 
lumi- 
nosity. 

Persistency 

curve 

590 

ad.  reading* 

Lumi- 
nosity 
curve. 

Absolute 
luminosity 
of  ex- 
tinction 
IV.  X  VI. 
48 

57 

6423 

1200 

1200 

•49 

5 

125 

56 

6330 

900 

850' 

'  -69 

7 

124 

55 

6242 

500 

550 

1-07 

10 

115 

54 

6152 

250 

250 

2-36 

17 

88 

53 

6074 

.. 

150 

3-93 

25 

78 

52 

5996 

90 

90 

6-56 

35 

^^ 

51 

5919 

60 

45 

13-1 

47 

44 

50 

5850 

27 

27 

21-8 

57 

32 

48 

5720 

18 

15 

39-3' 

m 

21 

46 

5596 

10 

10 

59 

69 

14 

44 

5481 

9-3 

8 

73-8 

64 

11 

42 

5373 

6-5 

6-2 

95-1 

56-5 

7 

40 

5270 

5-9 

5-9 

100 

45 

5-5 

38 

5172 

6 

6 

98-3 

32 

4 

36 

5085 

.. 

^'^ 

89-4 

20 

2-7 

35 

5043 

7 

7-2 

81-9 

16 

2-4 

.34 

5002 

.. 

8 

73-8 

12-5 

2-1 

-32 

4924 

10 

9-6 

61-5 

8 

1-6 

30 

4848 

11-5 

12 

49-2 

6 

1-5 

28 

4776 

14-5 

14-5 

40-7 

5 

1-5 

26 

4707 

20 

17-5 

33-7 

4 

1-5 

24 

4639 

20 

22 

26-8 

3 

1-4 

22 

4578 

.. 

30 

19-7 

2-4 

1-5 

18 

4459 

55 

57 

10-4 

1-3 

1-5 

14 

4349 

115 

115 

5-1 

•7 

1-7 

10 

4245 

.. 

160 

3-7 

'5 

1-7 

G 

4151 

200 

200 

2-9 

•4 

1-7 

c^24  :Appendix, 

—      '  Table  X.— V.  H.'s  Curves  (see  Fig.  33). 


Scale 
number. 


II. 


III. 


IV. 


Wave- 
length. 


Mean 

reading  of 

extinction  in 

millionths  of 

original 
luminosity* 


Adopted 
reading  in 
millionths 
of  original 
lumi- 
nosity. 


y. 


VI. 


Persistency 
curve 

530 

ad.  readiog* 


Lumi- 
nosity 


VII. 


Absolute 
luminosity 
of  ex- 
tinction 
IV.  X  VI. 
75       • 


67 

6423 

500 

500 

1-1 

31 

206 

56 

6330 

350 

350 

1-5 

43 

200 

54 

6152 

200 

180 

2-9 

61 

146-4 

52 

5996 

100 

100 

5-3 

70 

93-3 

50 

5850 

40 

40 

13-3 

73 

38^9 

48 

5720 

'i 

25 

21-2 

69 

23 

46 

5596 

10 

10 

53-0 

63 

8-4 

45 

5538 

6-5 

6-5 

81-6 

58 

5^0 

44 

5481 

6-0 

5-7 

93 

54 

4^1 

42 

5373 

5-5 

5-3 

100 

46 

3^3 

40 

5270 

5-5 

5-4 

98-2 

36 

2-6 

38 

5172 

5-7 

5-7 

93 

24 

1-8 

36 

5085 

6-7 

6-5 

81-6 

15 

1-3 

34 

5002 

7-0 

7-0 

75-7 

9-5 

•89 

32 

4924 

8-5 

8-5 

62-3 

7-0 

•79 

30 

4848 

10-7 

10-5 

50-5 

5*0 

•70 

28 

4776 

16 

16 

33-1 

3-7 

•79 

26 

4707 

.. 

22-5 

23-5 

2-7 

•81 

24 

4639 

30 

31 

17-1 

1-82 

•75 

22 

4578 

42-5 

42 

12-6 

1-4 

•78 

20 

4517 

55 

55 

9-6 

1-0 

•73 

16 

4404 

105 

100 

5-3 

•7 

•93 

12 

4296 

175 

170 

3-1 

•45 

1-02 

10 

4245 

200 

200 

2-7 

•34 

•91 

Appendix. 


225 


Table  XI.— B.  C.'s  Curves  (see  Fig.  34). 


I. 

XL 

iir. 

IV. 

V. 

\L 

Scale 
number. 

Wave- 
length. 

Adopted 

reading  in 

hundred 

thousandths. 

Persistency- 
curve 
12,500 
readings  in  V. 

Lumino- 
sity of 

original 
beam. 

Absolute 
luminosity 

of 
extinction 
III.  and  V. 

61 

6839 

7500 

1-6 

60 

6728 

5500 

2-3 

•5 

27-5 

59 

6622 

4000 

3.1 

1 

40      ' 

58 

6520 

2800 

4-5 

2 

56 

57 

6423 

2000 

6-2 

4 

80 

56 

6330 

1500 

8-3 

6 

90 

55 

6242 

1150 

10-8 

8 

92 

54 

6152 

950 

13-1 

11-5 

109-2 

53 

6074 

750 

16-6 

16 

120 

52 

5996 

580 

21-6 

21-5 

125 

51 

5919 

4S0 

29 

28-5 

122-5 

50 

5850 

350 

36 

37 

129-5 

49 

5783 

275 

45-5 

47 

129-2 

48 

5720 

215 

58 

60 

129 

47 

5658 

170 

73-4 

76 

129-2 

46 

5596 

140 

89-3 

92 

129 

45 

5538 

125 

100 

98 

122-5 

44 

5481 

125 

100 

100 

125 

43 

5427 

130 

96-1 

97 

126 

42 

5373 

150 

83 

85 

127-5 

41 

5321 

180 

69-4 

65 

117 

40 

5270 

215 

59 

45 

96-7 

39 

5221 

250 

50 

30 

75 

Q 


226 


Appendix, 
Table  XI. — continued. 


I. 

11. 

HI. 

IV. 

V. 

VI. 

Scale 
number. 

Wave- 
length. 

Adopted 
reading  in 
hundred 
thousandths. 

Persistency 

curve 

32,500 

readings  in  V. 

Lumino- 
sity of 
original 
beam. 

Absolute 
luminosity 

of 
extinction 
III.  and  V. 

38 

5172 

290 

43 

1-5 

723.2 

37 

5128 

335 

37 

16 

53-6 

36 

5055 

380 

33 

11-5 

43-7 

34 

5002 

500 

25 

7 

35 

32 

4994 

650 

19 

4 

26 

30 

4848 

850 

14 

2-5 

23-3 

28 

4776 

1100 

11-4 

2 

22 

26 

4707 

1500 

8-3 

1-5 

22 

24 

4639 

2000 

6-2 

1 

20 

22 

4578 

2700 

4-6 

5 

13-5 

18 

4459 

4750 

14 

4349 

7500 

10 

4245 

11000 

Appendix, 


227 


Table  XII. — M.'s  Luminosity  Curve  compared  with  the  Normal 

(see  Fig.  30). 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

Scale 
number. 

Wave- 
length. 

Mean 
reading. 

Mean 
reading 
X  1-8. 

Normal 
luminosity 

curve, 

centre  of 

eye. 

Difference 

of  last  two 

columns. 

Difference 
X  5-15. 

61 

6839 

2 

3-6 

4 

•4 

2-57 

59 

6621 

7 

12-6 

12-5 

—  •1 

•61 

57 

6423 

18 

32-4 

33 

+  •6 

3-09 

55 

6242 

36 

64-8 

65 

•2 

1-03 

53 

6074 

49 

88-2 

89-5 

1-3 

6-71 

52 

5996 

52 

95-4 

96-5 

1-1 

5-66 

51 

5919 

54 

97-2 

99-5 

2-3 

11-8 

50 

5850 

54 

97-2 

100 

2-8 

14-4 

49 

5782 

52-5 

94-5 

99-5 

5-0 

25-7 

48 

5720 

50 

90 

97 

7-0 

36-0 

47 

5658 

46 

82-8 

92-5 

9-7 

49-9 

46 

5596 

41 

73-8 

87 

13-2 

68-0 

44 

5481 

32 

57-6 

75 

17-4 

89 

42 

5373 

23 

43-2 

62.5 

19-3 

99 

40 

5270 

17 

30-6 

50 

19-4 

100 

38 

5172 

10 

17-5 

35-5 

18 

93 

36 

5085 

4 

7-2 

24 

16-8 

86-5 

34 

5002 

1-0 

1-8 

14-5 

12-7 

65-5 

31 

4885 

•5 

•7 

6-5 

5-8 

37-7 

28 

4776 

0 

0 

4 

4 

20-6 

228 


Appendix, 


Table  XIII.— Miss  W.'s  Curves  (see  Fig.  39). 


Scale 
number. 

Wave- 
length. 

Eeadings. 

Extinction  in 

TOO 000- 

Persistency 
curve. 

63 

7082 

0 

62 

6957 

1 

60 

6728 

7 

68 

6520 

18 

57 

6423 

28 

56 

6330 

43 

54 

6152 

76 

900 

2 

52 

5996 

90 

250 

7 

50 

5850 

95 

130 

13-5 

48 

5720 

93 

60 

29 

46 

5596 

83 

34 

61 

44 

54S1 

71 

22 

80 

42 

5321 

58 

18-5 

92 

40 

5270 

4. 

17-5 

100 

38 

5172 

32 

18 

94 

36 

5085 

21 

19-5 

90 

34 

5002 

12-5 

22 

79 

32 

4924 

7 

27 

65 

30 

4848 

4-5 

34 

51 

28 

4776 

3-0 

40 

38-5 

25 

4675 

1-5 

60 

29 

20 

4518 

0-4 

250 

7 

19 

4488 

0-0 

350 

6 

16 

4404 

— 

600 

(     229    ) 


INDEX 


Absorption  by  the  Yellow 

Spot 90 

Artificial  Spectrum  .  .  8;^ 
Cases  of  Defective  Colour 

Vision  unrecognised  .  67 
Clerk  Maxwell's  Colour- 
Box   42 

Clerk     Maxwell's     Colour 

Curves 47 

Colour,  and  the  Sensations 

required  to  produce  it  .  60 
Colour    Blindness   due   to 

Disease 137 

Colour-Blind  Persons  see  a 

Grey  in  the  Spectrum  .  05 
Colour  Discs  ....  82 
Colour  Fields  ....  18 
Colour  Matches  made  by 

the  Colour  Blind  .  .  70 
Colour  Patch  Apparatus  .  18 
Colour    Patch   Apparatus, 

Original  Form  of  .  .  19 
Comparison  of  the  Young 

and  Hering  Theory  .  189 
Complex  Colours  matched 

by  Simple  Colours  .  .  22 
Contrast  Colours    .      .      .     187 


164 

58 

57 


188 


11 


98 


Curious  Case  of  Congenital 
Colour  Blindness,  A 

Dalton  Colour  Blindness  . 

Daltonism,  or  Colour 
Blindness      .... 

Defective  Form  Yision 
connected  with  Colour 
Deficiency  due  to  Disease 

Definition  at  different  parts 
of  the  Eetina 

Enfeebled  Spectrum  Lu- 
minosity       .... 

Exhibiting  Colour  Blind- 
ness by  Colour  Discs      .       74 

Extinction  and  Persistency 
Curves  of  Green -Blind 
Persons 127 

Extinction  and  Persistency 
Curves  of  Monochro- 
matic Yision       .      .      .      125 

Extinction  and  Persistency 
Curves  of  Eed  -  Blind 
Persons 127 

Extinction  of  Colour    .      .     105 

Extinction  of  Light  by  the 
Centre  and  Periphery  of 
the  Eye 114 


230 


Index, 


Extinction    of    Colour    of 

Luminosity   of    the   Spec- 

equal Luminosity    . 

110 

trum  to  the  Colour  Blind 

81 

Extinction  of  Light  in  the 

Luminosity   of    the   Spec- 

Spectrum     .... 

109 

trum  to  the  IN'ormal  Eyed 

78 

Eye :    Explanation    of    its 

Malingerers,  Detection  of 

185 

Functions     .... 

3 

Matching  Colours  by  Mix- 

Fatigue of  the  Retina  . 

G,  30 

tures  of  Simple  Colours 

26 

Field  of  View  .... 

10 

Maxwell's  Colour  Equations 

202 

Fovea  Centralis 

4 

Maxwell's  Curves  for  Red 

Fundamental  Light     . 

34 

Blindness      .... 

69 

Green  -  Blind      Person's 

Measurement    of     Colour 

Description  of  the  Spec- 

Fields  

207 

trum,  A 

64 

Monochromatic  Vision  and 

Green    Monochromatic 

the  Spectrum      .     . 

66 

Vision 

131 

Number  of  Cones  in  the 

Helmholtz     Diagram     of 

Eye 

8 

Sensations     . 

38 

Optograms 

9 

Heredity  in  Colour  Blind- 

Pellet Tests      .... 

146 

ness  .      .           ... 

58 

Pendulum  Experiments     . 

36 

Hering's     Colour      Vision 

Persistency  Curves 

119 

Theory 

52 

Primary  Colours     . 

25 

Hering's  Theory  not  tri- 

Primary  Pigment  Colours 

27 

chromic 

57 

Progressive  Atrophy  of  the 

Holmgren's  Colour  Tests  . 

169 

Optic  Nerve 

153 

Koenig's  Colour  Sensation 

Purkinje's  Figures       .      . 

7 

Curves 

49 

Purples 

24 

Lissajou's  Figures  . 

37 

Red  and  Green  matched    . 

72 

Luminosity  of    the   Spec- 

Red-Blind Person's  Descrip- 

trum to  the  Centre  of 

tion  of  the  Spectrum,  A 

63 

the  Eye,  the  Fovea  Cen- 

Retina, Structure  of    . 

6 

tralis,   and    outside   the 

Retinal  Fatigue     .      .      . 

6,30 

Yellow  Spot       .      .      . 

88 

Rods  and  Cones      .      .      . 

8 

Luminosity   of    the   Spec- 

Seat of  Visual  Sensation    . 

7 

trum  to  partially  Colour 

Sensation  Curves  in  Terms 

Blind 

86 

of  Luminosity    . 

93 

Index. 


231 


Sensitiveness  of  the  Eye    .  121 

Simple  Colours       ...  17 
Simulation    of     Red    and 

Green  Blindness       .      .  175 
Spectrum  described  by  the 

Tobacco  Blind,  The       .  143 
Spectrum  Test  for  Colour 

Blindness      .      .      .      .  181 

Table  of  Wave-Lengths    .  17 

Tables 211 

Tobacco  Ambyopia      .      .  140 
Tobacco     Blindness,     Ex- 
amples of      ....  148 


Yiolet  Blindness     ...  73 
Yisifeility  of  an  Object  in 

light  of  different  Colours  123 

Visual  Purple   ....  9 
White   Monochromatic 

Yision 158 

Wool  Test,  The     .      .      .  170 

Yellow  Spot      ....  4 
Yellow   Spot   and   Colour 

Mixtures,  The    ...  28 
Young's  Theory,  Modifica- 
tion of 196 


Works  on  Photography 

BY 

Capt.  W.  de  W.  ABNEY,  C.B.,  D.C.L.,  F.R.S., 

Late  Royal  Engineers. 


Instruction  in  Photography.    Post  8vo.,  3s.  6^?. 

PnOTOaRAPHY   WITH  EMULSIONS.      Post  8V0.,  3s. 

Negative  Making.    Post  8vo.,  Is. 

Art  and  Practice  of  Silver  Printing.    Written 

in  conjunction  with  Mr.  H.  P.  Robinson.    Post  8vo  ,  2s.  Qd. 

Platinotype;    Written  in  conjunction  with  Lyonel 
Clabke.    Post  8vo.,  2s.  Qd. 


LONDON:   SAMPSON  LOW.  MARSTON  &  COMPANY,  LIMITED. 
St,  Dcnstan's  House,  Fetter  Laxe,  Fleet  Street,  E.C. 


LONDON:    PRINTED   BY   WILLIAM   CLOWES   AND   SONS,   LIMITED, 
STAMFORD   STREET  AND   CHARING  CROSS. 


3<^y 


